Nut can fit a mould (7)
I believe the answer is:
fanatic
'nut' is the definition.
(thesaurus)
'can fit a mould' is the wordplay.
'mould' indicates an anagram.
'can'+'fit'+'a'='canfita'
'canfita' anagrammed gives 'FANATIC'.
(Other definitions for fanatic that I've seen before include "Freak" , "enthusiastic" , "One who is excessively zealous" , "Narrow-minded enthusiast" , "Excessively devoted" .)