Tendency for a doctor to be unusually fit (5)
I believe the answer is:
drift
'tendency' is the definition.
(I know that drift is a type of tendency)
'doctor to be unusually fit' is the wordplay.
'doctor' becomes 'dr' (abbreviation).
'to be unusually' indicates an anagram.
'fit' anagrammed gives 'ift'.
'dr'+'ift'='DRIFT'
'for a' is the link.
(Other definitions for drift that I've seen before include "General idea (of snow?)" , "Float along as on waves" , "Move with the current" , "Float with current" , "move around without end" .)