Nearly fail to arrest criminal leader for petty crime (7)
I believe the answer is:
larceny
'petty crime' is the definition.
(I've seen this in another clue)
'nearly fail to arrest criminal leader' is the wordplay.
'fail' indicates an anagram (letters go wrong or fail).
'to arrest' is an insertion indicator (inserted letters are arrested or held).
'leader' says to take the initial letters.
The initial letter of 'criminal' is 'c'.
'nearly' anagrammed gives 'lareny'.
'lareny' enclosing 'c' is 'LARCENY'.
'for' is the link.
(Other definitions for larceny that I've seen before include "Legal term for theft" , "Theft of personal property" , "Stealing - theft" , "Burglary" , "Robbery" .)