Couple taking doctor to be a fool (5)
I believe the answer is:
dumbo
'a fool' is the definition.
(I've seen this before)
'couple taking doctor' is the wordplay.
'couple' becomes 'duo' (two people).
'taking' is an insertion indicator.
'doctor' becomes 'MB' (abbreviation for Bachelor of Medicine).
'duo' going around 'mb' is 'DUMBO'.
'to be' acts as a link.
(Other definitions for dumbo that I've seen before include "Disney film about an elephant" , "Disney cartoon elephant" , "idiot" , "Nineteen forty one Disney film" , "1941 Disney film" .)