Shopkeeper, perhaps, conventional in such things at the outset (8)
I believe the answer is:
stockist
'shopkeeper' is the definition.
'conventional in such things at the outset' is the wordplay.
'conventional' becomes 'stock' (as in stock phrases).
'at the outset' indicates taking the first letters (the outset is the beginning).
The first letters of 'in such things' is 'ist'.
'stock'+'ist'='STOCKIST'
'perhaps' is the link.
(Other definitions for stockist that I've seen before include "dealer" , "Retailer of a particular line of goods" , "Shop that carries a particular item" , "Retailer carrying a particular item" .)