Merchant in store at works (9)
I believe the answer is:
stationer
'merchant' is the definition.
(stationer is a kind of merchant)
'in store at works' is the wordplay.
'works' is an anagram indicator.
'in'+'store'+'at'='instoreat'
'instoreat' anagrammed gives 'STATIONER'.
(Other definitions for stationer that I've seen before include "Writing-materials shop" , "One assigning position" , "Seller of writing materials" , "Tears into (anag.)" , "Seller of paper, pens, etc" .)