Italian brother boxes leftover plants (5)
I believe the answer is:
flora
'plants' is the definition.
(I've seen this before)
'italian brother boxes leftover' is the wordplay.
'italian brother' becomes 'fra' (I've seen this in another clue).
'boxes' indicates putting letters inside (the outside letters must 'box' inside letters in).
'leftover' becomes 'LO' (abbreviations for left and over).
'fra' enclosing 'lo' is 'FLORA'.
(Other definitions for flora that I've seen before include "vegetation" , "__ MacDonald, heroine of the 1745 Jacobite rising" , "Various plants" , "growth" , "Plants of a particular region" .)