Customer certainly welcomes textile dealer (6)
I believe the answer is:
mercer
'textile dealer' is the definition.
(I've seen this in another clue)
'customer certainly welcomes' is the wordplay.
'welcomes' indicates a hidden word (I've seen 'welcoming' mean this).
'MERCER' can be found hidden inside 'customer certainly'.
(Other definitions for mercer that I've seen before include "Dealer in textiles or dry goods" , "Old tradesman" , "Dealer in textile fabrics" , "Supplier of fabrics" , "Textile dealer (historical)" .)