Tendency for a doctor to be unusually fit (5)
I believe the answer is:
'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'.
'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" .)