The Post Office again opening (4)
I believe the answer is:
pore
'opening' is the definition.
(pore is a kind of opening)
'the post office again' is the wordplay.
'the post office' becomes 'po' (abbreviation).
'again' becomes 're' (prefix meaning 'again').
'po'+'re'='PORE'
(Other definitions for pore that I've seen before include "Look intently" , "Rope around the skin-opening" , "Closely study" , "Small opening in the skin" , "Sweat-gland opening" .)