First of all, the phrase Durga Suktam
is not part of the text (Rig Veda).
The entire DurgA Suktam is found in the MahAnArAyana Upanishad (MNU), section 2 and not in the Vedas. It has a total of seven mantras in all.
And, of the seven mantras, only five are found in the Rig Veda, whilethe remaining two are not.
The five mantras that are from Rig Veda are:
jAtavedase sunavama somam, arAtiyato ni dahAti vedah, sa nah parshat ati durgAni.... naveva sindhum durita ati agnihi ||
....
to the one who knows all births, we prepare and offer the Soma; may he burn up completely the wealth and knowledge of our foes. May he lead us to happiness overcoming all states of grief; May Agni carry us as in a boart across a river preventing any wrong-doing or stumbling (durita).
Rig Veda 1.99.1
.....
Agne tvam pAraya navyo asmAn..... tokaya tanayaya sham yoh ||
...
O Agni, worthy of praise (navya), lead us beyond (ati) all difficulties (durgA) by happy means (svast); be awide and broad dwelling with many felicities for us. Grant peace and well-doing to our disciples and successors.
Rig Veda 1.189.2
etc.
The remaining three that i have not given (translations of) are Rig Veda 5.4.9 (vishvAni na durgahA...), Rig Veda 7.63.1 ( pritanajitam sahamAnam ugram..) and Rig Veda 8.11.10 ( pratno hi kam idyo adhvareshu....).
Now, all these mantras have Agni as their Devatas. So, they are dedicated to Agni.
Among the two mantras that are found in the aforementioned Upanishad i am giving here only one fully:
tAm agni varnAm tapasA jvalantim, vairochanim karma-phalesu jushtAm, durgAm devim sharnam aham prapadye, sutari tarase namaha ||
..........
I take refuge in Her, Goddess DurgA, who is with lustre of Agni and is radiant from askesis. She is the power belonging to the supreme (virochana) who manifests Himself manifoldy. She is the power in actions rendering their results efficious. You are skilled in saving; you take across the difficulties well. Salutations to you.
The other mantra from MNU gives the DurgA gAyatri mantra viz Om KatyAnaya Vidmahe kanyAkumari dhimahi tannoh Durgi prachodayath.
Om KatyAnaya Vidmahe kanyAkumari dhimahi tannoh Durgi prachodayath.
These seven mantras are popularly chanted together during Pujas as DurgA Suktam, although five of them are dedicated to Agni only.
Goddess DurgA is commonly interpreted as "one difficult to attain" or "one who overcomes all difficulties for the devotee".
In Vedas, Agni is often described as the leader in our journey towards perfection. And, the epithets to Goddess DurgA are very similar to those of Agni as mentioned in the given five Rig Veda mantras.