Rhubarb not initially ready to eat (4)
I believe the answer is:
ripe
'ready to eat' is the definition.
(I've seen this before)
'rhubarb not initially' is the wordplay.
'rhubarb' becomes 'tripe' (I've seen this before).
'not initially' means to remove the first letter.
'tripe' with its first letter taken away is 'RIPE'.
(Other definitions for ripe that I've seen before include "Ready for harvesting, in its prime" , "Opportune" , "'Fully grown, ready to be eaten (4)'" , "Ready (to be eaten or used)" , "Ready to harvest" .)