Snap if sweltering in Post Office (5)
I believe the answer is:
photo
'snap' is the definition.
(thesaurus)
'sweltering in post office' is the wordplay.
'sweltering' becomes 'hot' ('hot' can be similar in meaning to 'sweltering').
'in' is an insertion indicator.
'post office' becomes 'po'.
'hot' put into 'po' is 'PHOTO'.
'if' is the link.
(Other definitions for photo that I've seen before include "Camera picture" , "Snap - shot" , "Snap, exposure, in short" , "This race finish is very close" , "Picture, in short" .)