Small, her wager involves effervescent powder (7)
I believe the answer is:
sherbet
'effervescent powder' is the definition.
(I've seen this before)
'small her wager' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'wager' becomes 'bet' (synonyms).
's'+'her'+'bet'='SHERBET'
'involves' acts as a link.
This may not be right. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for sherbet that I've seen before include "Sweet fizzy, fruity, drink" , "Foreign beer" , "Sweet effervescent powder" , "Fizzy stuff" , "Fizzer'" .)