Post Office seizing sexy image (5)
I believe the answer is:
photo
'image' is the definition.
(photo is a kind of image)
'post office seizing sexy' is the wordplay.
'post office' becomes 'po'.
'seizing' indicates putting letters inside.
'sexy' becomes 'hot' ('hot' can be similar in meaning to 'sexy').
'po' going around 'hot' is 'PHOTO'.
(Other definitions for photo that I've seen before include "This race finish is very close" , "In brief, a snapshot" , "Camera image" , "Still" , "Snap (abbrev.)" .)