Organise a retailer's first line (7)
I believe the answer is:
arrange
'organise' is the definition.
(arranging is a kind of organizing)
'a retailer's first line' is the wordplay.
'retailer's first' becomes 'r' (1st letter of 'retailer').
'line' becomes 'range' (I've seen this before).
'a'+'r'+'range'='ARRANGE'
(Other definitions for arrange that I've seen before include "Orchestrate - order" , "Put into a proper order" , "Set in place" , "Dispose" , "Rare nag (anag.)" .)