Rough Irish customer, a frequent visitor (9)
I believe the answer is:
irregular
'rough' is the definition.
(I've seen this before)
'irish customer a frequent visitor' is the wordplay.
'irish' becomes 'ir' (abbreviation).
'customer a frequent visitor' becomes 'regular' (as in a regular customer).
'ir'+'regular'='IRREGULAR'
(Other definitions for irregular that I've seen before include "unprincipled" , "Unconventional" , "Out of the ordinary" , "Unsymmetrical; soldier" , "Disorderly, uneven" .)