Grammatical parity? Yes, of course! (9)

I believe the answer is:

agreement

I'm a little stuck... Click here to teach me more about this clue!

'grammatical parity?' is the definition.
'agreement' can be an answer for 'parity?' (thesaurus). I am not certain of the 'grammatical' bit.

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for agreement that I've seen before include "Accord, contract" , "Harmony, concurrence" , "Contract or promise" , "deal" , "consensus" .)

I've seen this clue in The Guardian.

Want a hint initially instead of a full solution? Install my app