Tenor made unusually fit by a doctor (5)
I believe the answer is:
drift
'tenor' is the definition.
(I know that Drift is a type of tenor)
'unusually fit by a doctor' is the wordplay.
'unusually' is an anagram indicator.
'by' is a charade indicator (letters next to each other).
'a doctor' becomes 'dr' (abbreviation).
'fit' anagrammed gives 'ift'.
'ift' put after 'dr' is 'DRIFT'.
'made' acts as a link.
(Other definitions for drift that I've seen before include "move around without end" , "Trend; meaning" , "Large heap (of snow)" , "Slide" , "Import" .)