Grew mad about a bet (5)
I believe the answer is:
wager
'bet' is the definition.
(I know that bet can be written as wager)
'grew mad about a' is the wordplay.
'mad' indicates anagramming the letters.
'about' is an insertion indicator.
'grew' anagrammed gives 'wger'.
'wger' going around 'a' is 'WAGER'.
(Other definitions for wager that I've seen before include "better to put this on?" , "Put on a bit" , "stake required" , "(Place a) stake" , "A bet or stake" .)