Downed one can at end of month (6)
I believe the answer is:
martin
'downed one' is the definition.
'can at end of month' is the wordplay.
'can' becomes 'tin' (synonyms).
'at end of' means one lot of letters go next to another (some letters go at the end of others).
'month' becomes 'Mar' (short for March).
'tin' after 'mar' is 'MARTIN'.
(Other definitions for martin that I've seen before include "Small swallow" , "-- Chuzzlewit (Dickens)" , "Man" , "Dickens novel partly set in America" , "Short-tailed swallow" .)