Forcibly enter Irish port after game (6)
I believe the answer is:
irrupt
'forcibly enter' is the definition.
(to irrupt is to enter forcibly)
'irish port after game' is the wordplay.
'irish' becomes 'IR' (abbreviation).
'port' becomes 'pt' (abbreviation).
'after' is a charade indicator (letters next to each other).
'game' becomes 'RU' (rugby union).
'pt' after 'ru' is 'rupt'.
'ir'+'rupt'='IRRUPT'
(Other definitions for irrupt that I've seen before include "Enter forcibly or violently" , "Break, rush, in" , "Enter forcibly and suddenly" .)