I had sex appeal to ensnare old fool (5)
I believe the answer is:
idiot
'fool' is the definition.
(thesaurus)
'i had sex appeal to ensnare old' is the wordplay.
'i had' becomes 'id' (I'd).
'sex appeal' becomes 'it'.
'to ensnare' indicates putting letters inside.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'it' placed around 'o' is 'iot'.
'id'+'iot'='IDIOT'
(Other definitions for idiot that I've seen before include "Mug" , "Dostoevsky hero" , "Stupid person" , "nutter" , "Twit" .)