Still very warm in Post Office (5)
I believe the answer is:
photo
'still' is the definition.
(I've seen this before)
'very warm in post office' is the wordplay.
'very warm' becomes 'hot' (I've seen this before).
'in' means one lot of letters goes inside another.
'post office' becomes 'po' (abbreviation).
'hot' going within 'po' is 'PHOTO'.
(Other definitions for photo that I've seen before include "Picture (abbr)" , "Snap, picture" , "Camera image" , "Snap - shot" , "Camera picture" .)