Opera singer somewhat fat? Enormous! (5)
I believe the answer is:
tenor
'opera singer' is the definition.
(thesaurus)
'somewhat fat? enormous' is the wordplay.
'somewhat' indicates a hidden word.
'TENOR' is hidden within 'fat enormous'.
(Other definitions for tenor that I've seen before include "Drift of thought" , "General drift or meaning" , "theme" , "exact words" , "character" .)