Tell niece about customers (9)
I believe the answer is:
clientele
'customers' is the definition.
(I've seen this in another clue)
'tell niece about' is the wordplay.
'about' is an anagram indicator.
'tell'+'niece'='tellniece'
'tellniece' anagrammed gives 'CLIENTELE'.
(Other definitions for clientele that I've seen before include "All one's customers" , "They patronise" , "Patronising people" , "Customers (of a shop)" , "Customers of a business" .)