Sell a litre somehow (6)
I believe the answer is:
retail
'sell' is the definition.
(retailing is a kind of selling)
'a litre somehow' is the wordplay.
'somehow' indicates an anagram.
'a'+'litre'='alitre'
'alitre' anagrammed gives 'RETAIL'.
(Other definitions for retail that I've seen before include "This trade sells direct to the public" , "High street selling" , "Narrate or sell to public" , "Sell (in shop)" , "This trade sells to the general public" .)