Customers tell niece off (9)
I believe the answer is:
clientele
'customers' is the definition.
(I've seen this before)
'tell niece off' is the wordplay.
'off' is an anagram indicator.
'tell'+'niece'='tellniece'
'tellniece' with letters rearranged gives 'CLIENTELE'.
(Other definitions for clientele that I've seen before include "A shop's customers" , "All one's customers" , "Patrons" , "Market" , "Customers, collectively" .)