Hard drinker knocked back moonshine (4)
I believe the answer is:
tosh
'moonshine' is the definition.
(I've seen this before)
'hard drinker knocked back' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'drinker' becomes 'sot' (sot is a kind of drinker).
'knocked back' says the letters should be written in reverse.
'h'+'sot'='hsot'
'hsot' written backwards gives 'TOSH'.
(Other definitions for tosh that I've seen before include "Rubbish, nonsense" , "Pal" , "moonshine" , "Nonsense (colloq.)" , "Boloney" .)