Show-off in post office regrets turning up (6)
I believe the answer is:
poseur
'show-off' is the definition.
(I know that poseur is a type of show-off)
'post office regrets turning up' is the wordplay.
'post office' becomes 'po' (abbreviation).
'regrets' becomes 'rues' ('rue' can be a synonym of 'regret').
'turning up' shows that the letters should be reversed in order.
'rues' in reverse letter order is 'seur'.
'po'+'seur'='POSEUR'
'in' acts as a link.
(Other definitions for poseur that I've seen before include "One trying to impress others" , "Exhibitionist" , "One shows off to impress" , "One who strikes attitudes, a phoney" , "Pretentious type" .)