In report, he mentioned subject (5)
I believe the answer is:
theme
'subject' is the definition.
(theme can mean a topic or subject)
'in report he mentioned' is the wordplay.
'in' says the answer is hidden in the clue.
'THEME' is hidden in the letters of 'report he mentioned'.
(Other definitions for theme that I've seen before include "Recurring melody" , "Burden" , "Main idea or topic" , "Topical item" , "Leitmotiv" .)