Disorderly Irish customer, a familiar one (9)
I believe the answer is:
irregular
'disorderly' is the definition.
(thesaurus)
'irish customer a familiar one' is the wordplay.
'irish' becomes 'ir' (abbreviation).
'customer a familiar one' becomes 'regular' (as in a regular customer).
'ir'+'regular'='IRREGULAR'
(Other definitions for irregular that I've seen before include "Erratic" , "Not uniform" , "Out of order" , "Not even or balanced" , "unprincipled" .)