Reserved, and put in, fizzy drink (6)
I believe the answer is:
shandy
'fizzy drink' is the definition.
(fizzy alcoholic drink)
'reserved and put in' is the wordplay.
'reserved' becomes 'shy' (I've seen this before).
'put in' is an insertion indicator.
'shy' enclosing 'and' is 'SHANDY'.
(Other definitions for shandy that I've seen before include "A misture of ale and lemonade" , "Lemonade-beer drink" , "Beer and lemonade mixture" , "Alcohol" , "mixed drink" .)