Tree line crossed by tailless feline (4)
I believe the answer is:
plum
'tree' is the definition.
(I know that plum is a type of fruit tree)
'line crossed by tailless feline' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'crossed by' means one lot of letters goes inside another (inserted letters are crossed or covered by others).
'tailless' means to remove the last letter (the tail of the word is removed).
'feline' becomes 'puma' (puma is a kind of feline).
'puma' with its final letter taken off is 'pum'.
'l' inserted into 'pum' is 'PLUM'.
(Other definitions for plum that I've seen before include "Edible oval fruit" , "murderer" , "Select" , "Dark-red fruit" , "Lump of fruit" .)