Charlie, on one occasion, exercises theory (7)
I believe the answer is:
concept
'theory' is the definition.
(thesaurus)
'charlie on one occasion exercises' is the wordplay.
'charlie' becomes 'c' (phonetic alphabet: alpha, bravo, charlie etc.).
'on one occasion' becomes 'once' ('once' can be a synonym of 'on one occasion').
'exercises' becomes 'pt'.
'c'+'once'+'pt'='CONCEPT'
(Other definitions for concept that I've seen before include "Abstract idea; mental picture" , "Product of the mind" , "Notion" , "Abstract or general idea" .)