Plant leaf half destroyed by insect (5)
I believe the answer is:
bugle
'plant' is the definition.
(also known as bugleweed)
'leaf half destroyed by insect' is the wordplay.
'half destroyed' means to take half the letters.
'by' is a charade indicator (letters next to each other).
'insect' becomes 'bug' (bug is a kind of insect).
'leaf' halved is 'le'.
'le' put after 'bug' is 'BUGLE'.
(Other definitions for bugle that I've seen before include "Instrument used for military signals" , "It calls to soldiers" , "Horn" , "Small trumpet; plant" , "one may have blown it!" .)