Wheeze while visiting doctor (4)
I believe the answer is:
gasp
'wheeze' is the definition.
(I've seen this in another clue)
'while visiting doctor' is the wordplay.
'while' becomes 'as' (eg 'while I walked' means 'as I walked').
'visiting' indicates putting letters inside.
'doctor' becomes 'gp' (general practitioner).
'as' put inside 'gp' is 'GASP'.
(Other definitions for gasp that I've seen before include "Fight for breath" , "Have trouble breathing" , "Sudden intake of breath" , "Catch one's breath in astonishment" , "Breathe irregularly" .)