Son having trouble knocked back drink (4)
I believe the answer is:
soda
'drink' is the definition.
(I know that soda is a type of Non-alcoholic drink)
'son having trouble knocked back' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'having' says to put letters next to each other.
'trouble' becomes 'ado' (both can mean bother or commotion).
'knocked back' shows that the letters should be reversed in order.
'ado' reversed gives 'oda'.
's'+'oda'='SODA'
(Other definitions for soda that I've seen before include "for use with whisky?" , "Variety of pop" , "kind of water" , "Accompaniment to whisky" , "fizz" .)