Small fruit is weapon (5)
I believe the answer is:
spear
'is weapon' is the definition.
(I've seen this before)
'small fruit' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'fruit' becomes 'pear' (pear is a kind of fruit).
's'+'pear'='SPEAR'
(Other definitions for spear that I've seen before include "Long pointed weapon" , "Javelin or harpoon" , "Impale" , "stick" , "Thrown weapon" .)