To compile difficult? Nothing in it! (5)
I believe the answer is:
hoard
'to compile' is the definition.
('hoard' can be a synonym of 'compile')
'difficult? nothing in it' is the wordplay.
'difficult?' becomes 'hard' ('hard' can be a synonym of 'difficult').
'nothing' becomes 'o' (looks like zero - 0).
'in it' is an insertion indicator.
'hard' placed around 'o' is 'HOARD'.
(Other definitions for hoard that I've seen before include "Hard kick" , "Secret store of valuables" , "Accumulation" , "One's savings" , "Secret stash" .)