Guard Irishman with posh car? (9)
I believe the answer is:
patroller
'guard' is the definition.
(patroller is a kind of guard)
'irishman with posh car?' is the wordplay.
'irishman' becomes 'pat'.
'with' means one lot of letters go next to another.
'posh car?' becomes 'roller'.
'pat'+'roller'='PATROLLER'
(Other definitions for patroller that I've seen before include "Security guard" , "perambulator" , "Mobile guard" , "Beach guard, perhaps" , "sentinel" .)