Or is he mistaken, this shopkeeper? (6)
I believe the answer is:
hosier
'this shopkeeper?' is the definition.
(hosier is a kind of shopkeeper)
'or is he mistaken' is the wordplay.
'mistaken' is an anagram indicator.
'or'+'is'+'he'='orishe'
'orishe' with letters rearranged gives 'HOSIER'.
(Other definitions for hosier that I've seen before include "Manufacturer or seller of stockings" , "Seller of stockings etc" , "seller of 24 [NYLONS]" , "Shopkeeper who sells stockings" , "Maker or dealer in stockings" .)