Fancy Head of Intelligence cracked Enigma (7)
I believe the answer is:
imagine
'fancy' is the definition.
(thesaurus)
'head of intelligence cracked enigma' is the wordplay.
'head of' suggests taking the first letters.
'cracked' indicates an anagram.
The initial letter of 'intelligence' is 'i'.
'enigma' is an anagram of 'magine'.
'i'+'magine'='IMAGINE'
(Other definitions for imagine that I've seen before include "Picture in one's mind" , "Fancy - dream" , "A Gemini? (anag)" , "Conjure up in mind,fancy" , "Picture, conceive" .)