Son is going to drink a lot (5)
I believe the answer is:
swill
'drink a lot' is the definition.
(swill can mean to have a big drink)
'son is going to' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'is going to' becomes 'will' (John is going to eat/John will eat).
's'+'will'='SWILL'
(Other definitions for swill that I've seen before include "drink heavily" , "Pigs' food" , "Wet feed for pigs" , "Pig-food; rinse" , "Hogwash" .)