First bit of stomach fits into opening? Let out pants (4)
I believe the answer is:
gasp
'let out pants' is the definition.
(to gasp is to pant or breathe heavily)
'first bit of stomach fits into opening?' is the wordplay.
'first bit of stomach' becomes 's' (gst letter of 'stomach').
'fits into' means one lot of letters goes inside another.
'opening?' becomes 'gap' (gap is a kind of opening).
's' put inside 'gap' is 'GASP'.
(Other definitions for gasp that I've seen before include "Struggle for breath with open mouth" , "Short sharp intake of breath" , "Breathe sharply in amazement, horror etc" , "Fight for breath" , "Puff, struggle for breath" .)