Dealer in customer cereals (6)
I believe the answer is:
mercer
'dealer' is the definition.
(I know that mercer is a type of dealer)
'in customer cereals' is the wordplay.
'in' indicates a hidden word.
'MERCER' can be found hidden inside 'customer cereals'.
(Other definitions for mercer that I've seen before include "Dealer in textile fabrics" , "Dealer in fine cloth" , "Man of the cloth" , "Old tradesman" , "Dealer in textiles or dry goods" .)