The fool has a fad about unusual heat (7)
I believe the answer is:
fathead
'the fool' is the definition.
(fathead is a kind of fool)
'a fad about unusual heat' is the wordplay.
'about' indicates putting letters inside.
'unusual' indicates an anagram.
'heat' anagrammed gives 'thea'.
'fad' enclosing 'thea' is 'FATHEAD'.
'has' is the link.
(Other definitions for fathead that I've seen before include "Fool" , "silly ass!" , "Charlie" , "Slang idiot with enlarged cranium?" , "Dimwit" .)