Corresponded about grey seals (6)
I believe the answer is:
agreed
'corresponded' is the definition.
('agree' can be a synonym of 'correspond')
'about grey seals' is the wordplay.
I cannot quite see how this works, but
'about' could be 're' (regarding) and 're' is found within the answer.
The remaining letters 'aged' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for agreed that I've seen before include "Saw eye to eye" , "Was consistent" , "That's right" , "Settled by common consent" , "Consented, settled" .)