Breathe in sharply when visiting doctor (4)
I believe the answer is:
gasp
'breathe in sharply' is the definition.
(I've seen this before)
'when visiting doctor' is the wordplay.
'when' becomes 'as'.
'visiting' is an insertion indicator.
'doctor' becomes 'gp' (general practitioner**).
'as' placed into 'gp' is 'GASP'.
(Other definitions for gasp that I've seen before include "Have trouble breathing" , "Say "ah"" , "Convulsive breath" , "Gaps in struggle for breath" , "Sound of astonishment" .)