Square packing crate designed for fruit (9)
I believe the answer is:
nectarine
'for fruit' is the definition.
(nectarine is a kind of fruit)
'square packing crate designed' is the wordplay.
'square' becomes 'nine' (nine is a square number).
'packing' means one lot of letters goes inside another (some letters are packed inside).
'designed' is an anagram indicator (designed with the letters).
'crate' is an anagram of 'ectar'.
'nine' going around 'ectar' is 'NECTARINE'.
(Other definitions for nectarine that I've seen before include "Fruit - aren't nice (anag)" , "creating a sort of jam" , "fruity" , "type of tree" , "Juicy fruit" .)