Firm fruit (5)
I believe the answer is:
apple
This clue could be a double definition.
'firm' is the first definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'fruit' is the second definition.
(I know that apple is a type of fruit of Maloideae)
Can you help me to learn more?
(Other definitions for apple that I've seen before include "The - Cart (Shaw)" , "Pomaceous fruit" , "US computer company" , "Fruit used in cider" , "eg Cox, Bramley" .)