Sharp intake of breath when pinched by doctor (4)
I believe the answer is:
gasp
'sharp intake of breath' is the definition.
(I've seen this before)
'when pinched by doctor' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'pinched by' means one lot of letters goes inside another (inserted letters get pinched or taken).
'doctor' becomes 'gp' (general practitioner).
'as' put within 'gp' is 'GASP'.
(Other definitions for gasp that I've seen before include "Breathe irregularly" , "Say "ah"" , "Utter breathlessly" , "Convulsive breath" , "Breathe sharply in amazement, horror etc" .)