I had the thing to capture old fool (5)
I believe the answer is:
idiot
'fool' is the definition.
(thesaurus)
'i had the thing to capture old' is the wordplay.
'i had' becomes 'id' (I'd).
'the thing' becomes 'it'.
'to capture' indicates putting letters inside.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'it' enclosing 'o' is 'iot'.
'id'+'iot'='IDIOT'
(Other definitions for idiot that I've seen before include "Dunce" , "Dope" , "Fool; half-wit" , "Jackass" , "Village character" .)