Thief finally struck wildly at policeman (12)
I believe the answer is:
kleptomaniac
'thief' is the definition.
(kleptomaniac is a kind of thief)
'finally struck wildly at policeman' is the wordplay.
'finally' says to take the final letters.
'wildly' indicates an anagram.
The last letter of 'struck' is 'k'.
'at'+'policeman'='atpoliceman'
'atpoliceman' with letters rearranged gives 'leptomaniac'.
'k'+'leptomaniac'='KLEPTOMANIAC'
(Other definitions for kleptomaniac that I've seen before include "Obsessive thief" , "Motiveless thief" , "A man to pickle (anag) - one with an urge to steal" , "Compulsive thief" .)