Agreement to do something properly (4,4)
I believe the answer is:
very well
'properly' is the definition.
Both the definition and answer are adverbs. Perhaps you can see an association between them that I don't see?
'agreement to do something' is the wordplay.
I cannot really see how this works, but
'something' could be 'er' (I've seen this in other clues) and 'er' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for very well that I've seen before include "If you say so - in extremely good health" , "Blooming" , "In the pink - all right then" , "Ok, as you wish - sounds good" , "Extremely healthy - phrase signifying assent (sometimes ironically)" .)