Put a diet off, showing cleverness (8)
I believe the answer is:
aptitude
'cleverness' is the definition.
(thesaurus)
'put a diet off' is the wordplay.
'off' indicates anagramming the letters.
'put'+'a'+'diet'='putadiet'
'putadiet' with letters rearranged gives 'APTITUDE'.
'showing' acts as a link.
(Other definitions for aptitude that I've seen before include "capacity" , "fitness" , "Update it, having a relevant talent" , "Bent" , "Update it, having natural skill" .)