It's art, composed around single retailer (9)
I believe the answer is:
stationer
'retailer' is the definition.
(I know that stationer is a type of retailer)
'it's art composed around single' is the wordplay.
'composed' indicates an anagram.
'around' means one lot of letters goes inside another.
'single' becomes 'one' (a single thing - only one).
'its'+'art'='itsart'
'itsart' anagrammed gives 'statir'.
'statir' enclosing 'one' is 'STATIONER'.
(Other definitions for stationer that I've seen before include "One assigning position" , "Retailer of writing materials etc" , "Poster" , "has what writer needs" , "Shopkeeper" .)