Bet inn air could become intoxicating (9)
I believe the answer is:
inebriant
'intoxicating' is the definition.
(I've seen this before)
'bet inn air could become' is the wordplay.
'could become' indicates anagramming the letters.
'bet'+'inn'+'air'='betinnair'
'betinnair' anagrammed gives 'INEBRIANT'.
(Other definitions for inebriant that I've seen before include "Heady" , "Whisky, for example" , "drunk - on this?" , "Intoxicating" .)