Acid fruit (4)
I believe the answer is:
lime
'acid fruit' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for lime that I've seen before include "Type of stone" , "Round green fruit or a linden tree" , "Kind of tree or substance used in cement" , "Green fruit -- green colour" , "Bright light green" .)