Snap when sweltering in post office (5)
I believe the answer is:
photo
'snap' is the definition.
(I've seen this before)
'sweltering in post office' is the wordplay.
'sweltering' becomes 'hot' (similar in meaning).
'in' indicates putting letters inside.
'post office' becomes 'po' (abbreviation).
'hot' placed within 'po' is 'PHOTO'.
'when' is the link.
(Other definitions for photo that I've seen before include "Still" , "Camera picture" , "Camera image" , "Snap (abbrev.)" , "In brief, a snapshot" .)