Too keen again, agree to change (9)
I believe the answer is:
overeager
'too keen' is the definition.
(I've seen this before)
'again agree to change' is the wordplay.
'again' becomes 'over' (as in the phrases 'start over' and 'start again').
'to change' indicates an anagram.
'agree' with letters rearranged gives 'eager'.
'over'+'eager'='OVEREAGER'
(Other definitions for overeager that I've seen before include "too anxious to please?" , "Excessively keen" , "with enthusiasm" .)