Meat market: drain running back alongside one (8)
I believe the answer is:
pastrami
'meat' is the definition.
(I know this)
'market drain running back alongside one' is the wordplay.
'market' becomes 'mart' (synonyms).
'drain' becomes 'sap' (synonyms).
'running back' shows that the letters should be reversed in order.
'alongside' is a charade indicator (letters next to each other).
'one' becomes 'i' (Roman numeral).
'mart'+'sap'='martsap'
'martsap' back-to-front is 'pastram'.
'pastram'+'i'='PASTRAMI'
(Other definitions for pastrami that I've seen before include "Seasoned smoked beef, served in thin slices" , "Sliced cured beef" , "beef dish" , "smoked joint" , "Highly seasoned cut of smoked beef" .)