Tenor made unusually fit by a doctor (5)
I believe the answer is:
'tenor' is the definition.
(I've seen this before)
'unusually fit by a doctor' is the wordplay.
'unusually' indicates anagramming the letters.
'by' says to put letters next to each other.
'a doctor' becomes 'dr' ('Dr' can be a synonym of 'doctor'**).
'fit' with letters rearranged gives 'ift'.
'ift' 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" .)