Son gets drunk over casual fling (4)
I believe the answer is:
toss
'casual fling' is the definition.
(to toss something is to fling it casually)
'son gets drunk over' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'gets' means one lot of letters go next to another.
'drunk' becomes 'sot' (sot can mean a drunken person).
'over' shows that the letters should be reversed in order.
's'+'sot'='ssot'
'ssot' back-to-front is 'TOSS'.
(Other definitions for toss that I've seen before include "cast" , "Fling" , "Raise a little money" , "Careless not to give this" , "decision-making process" .)