Type of stone fruit (4)
I believe the answer is:
lime
This clue is a double definition.
'type of stone' is the first definition.
(I've seen this before)
'fruit' is the second definition.
(I know that lime is a type of fruit)
(Other definitions for lime that I've seen before include "Mailed item" , "Yellowish-green" , "Tree, linden or citrus" , "Round green fruit or a linden tree" , "Citrus fruit - shade of green" .)