American uncle knocked back first of home brew (4)
I believe the answer is:
mash
'brew' is the definition.
(I've seen this before)
'american uncle knocked back first of home' is the wordplay.
'american uncle' becomes 'sam' (Uncle Sam is a nickname for the USA).
'knocked back' shows that the letters should be reversed in order.
'first of home' becomes 'h' (sst letter of 'home').
'sam' back-to-front is 'mas'.
'mas'+'h'='MASH'
(Other definitions for mash that I've seen before include "Pummel" , "food for animals" , "Bangers' frequent partner?" , "Pureed potato" , "Pulverise" .)