Ready reckoner is proof enough -- initially (4)
I believe the answer is:
ripe
'ready' is the definition.
('ripe' can be similar in meaning to 'ready')
'reckoner is proof enough initially' is the wordplay.
'initially' suggests taking the first letters.
The initial letters of 'reckoner is proof enough' is 'RIPE'.
(Other definitions for ripe that I've seen before include "'Fully grown, ready to be eaten (4)'" , "green? No!" , "Fully developed or matured" , "Ready to pick" , "Fully grown, ready for eating" .)