Sell a litre wrongly (6)
I believe the answer is:
retail
'sell' is the definition.
(I know that retail is a more specific form of sell)
'a litre wrongly' is the wordplay.
'wrongly' indicates anagramming the letters.
'a'+'litre'='alitre'
'alitre' is an anagram of 'RETAIL'.
(Other definitions for retail that I've seen before include "Sell direct to the customer" , "Sell (in shop)" , "Flog" , "report" , "Trade selling goods to the general public" .)