A diet put out for fitness (8)
I believe the answer is:
aptitude
'fitness' is the definition.
(thesaurus)
'a diet put out' is the wordplay.
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'diet'+'put'='dietput'
'dietput' is an anagram of 'ptitude'.
'a'+'ptitude'='APTITUDE'
'for' is the link.
(Other definitions for aptitude that I've seen before include "Natural ability or skill" , "fitness" , "A particular talent or facility" , "Update it, having a relevant talent" , "Bent" .)