Son - stir flipping drink! (4)
I believe the answer is:
soda
'drink' is the definition.
(soda is a kind of drink)
'son stir flipping' is the wordplay.
'son' becomes 's'.
'stir' becomes 'ado' (synonyms).
'flipping' says the letters should be written backwards.
'ado' in reverse letter order is 'oda'.
's'+'oda'='SODA'
(Other definitions for soda that I've seen before include "fizz" , "pressurised liquid" , "Variety of pop" , "Soft drink" , "kind of water" .)