Sell about a litre (6)
I believe the answer is:
retail
'sell' is the definition.
(I know that retail is a more specific form of the action sell)
'about a litre' is the wordplay.
'about' indicates an anagram.
'a'+'litre'='alitre'
'alitre' anagrammed gives 'RETAIL'.
(Other definitions for retail that I've seen before include "Narrate or sell to public" , "Tell story" , "Sale of goods individually" , "Sell (goods) to the public" , "Trade selling goods to the general public" .)