Fitness update's modified to include it (8)
I believe the answer is:
aptitude
'fitness' is the definition.
(thesaurus)
'update's modified to include it' is the wordplay.
'modified' indicates an anagram.
'to include' is an insertion indicator.
'update' with letters rearranged gives 'aptude'.
'aptude' going around 'it' is 'APTITUDE'.
(Other definitions for aptitude that I've seen before include "A particular talent or facility" , "Natural gift" , "Update it, having natural skill" , "Update it, having a relevant talent" , "A natural talent or skill, for which there may be a test" .)