Son with difficulty rejected fizzy drink (4)
I believe the answer is:
soda
'fizzy drink' is the definition.
(soda is a fizzy drink)
'son with difficulty rejected' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'with' says to put letters next to each other.
'difficulty' becomes 'ado' (ado is bother or difficulty).
'rejected' is a reversal indicator.
'ado' backwards is 'oda'.
's'+'oda'='SODA'
(Other definitions for soda that I've seen before include "fizz" , "pressurised liquid" , ""Splashed" drinks mixer" , "Accompaniment to whisky" , "Sodium carbonate" .)