Post Office holding very popular image (5)
I believe the answer is:
photo
'image' is the definition.
(photo is a kind of image)
'post office holding very popular' is the wordplay.
'post office' becomes 'po' (abbreviation).
'holding' is an insertion indicator.
'very popular' becomes 'hot' (I've seen this before).
'po' enclosing 'hot' is 'PHOTO'.
(Other definitions for photo that I've seen before include "In brief, a snapshot" , "Camera picture" , "Snap, exposure, in short" , "Camera image" , "Snap, picture" .)