Special activity backing drink (4)
I believe the answer is:
soda
'drink' is the definition.
(soda is a kind of drink)
'special activity backing' is the wordplay.
'special' becomes 'S' (common abbreviation as in Special Air Service).
'activity' becomes 'ado' (ado can mean commotion or activity).
'backing' says the letters should be written in reverse.
'ado' back-to-front is 'oda'.
's'+'oda'='SODA'
(Other definitions for soda that I've seen before include "Mixer drink" , "Variety of pop" , "Accompaniment to whisky" , "fizz" , "kind of water" .)