Mature response is particularly effective, initially (4)
I believe the answer is:
ripe
'mature' is the definition.
(I know that mature can be written as ripe)
'response is particularly effective initially' is the wordplay.
'initially' says to take the initial letters.
The first letters of 'response is particularly effective' is 'RIPE'.
(Other definitions for ripe that I've seen before include "'Fully grown, ready to be eaten (4)'" , "Ready (to be eaten or used)" , "Opportune" , "Seasoned, fully grown" , "Fully developed or matured" .)