Fruit crate broken — it’s sort of square outside (9)
I believe the answer is:
nectarine
'fruit' is the definition.
(nectarine is a kind of fruit)
'crate broken it's sort of square outside' is the wordplay.
'broken' is an anagram indicator.
'it's sort of square' becomes 'nine' (I can't explain this - if you can you should give a lot more credence to this answer).
'outside' means one lot of letters goes inside another.
'crate' with letters rearranged gives 'ectar'.
'ectar' placed within 'nine' is 'NECTARINE'.
Can you help me to learn more?
(Other definitions for nectarine that I've seen before include "Kind of peach with thin skin and no down" , "Juicy fruit" , "Trace nine to a kind of peach" , "Variety or mutation of a peach" , "type of tree" .)