Sharp intake of breath while visiting doctor (4)
I believe the answer is:
gasp
'sharp intake of breath' is the definition.
(I've seen this before)
'while visiting doctor' is the wordplay.
'while' becomes 'as' (eg 'while I walked' means 'as I walked').
'visiting' means one lot of letters goes inside another.
'doctor' becomes 'gp' (general practitioner).
'as' inserted into 'gp' is 'GASP'.
(Other definitions for gasp that I've seen before include "Utter breathlessly" , "Puff, struggle for breath" , "Gaps in struggle for breath" , "Convulsive breath" , "Breathe sharply in amazement, horror etc" .)