Bully boy started argument about a bet (8)
I believe the answer is:
browbeat
'bully' is the definition.
(I've seen this before)
'boy started argument about a bet' is the wordplay.
'started' indicates taking the first letters.
'argument' becomes 'row' (I've seen this before).
'about' means one lot of letters goes inside another.
The initial letter of 'boy' is 'b'.
'a' placed inside 'bet' is 'beat'.
'b'+'row'+'beat'='BROWBEAT'
(Other definitions for browbeat that I've seen before include "Hector" , "Intimidate with words" , "Bully verbally" , "Bully or intimidate - using forehead?" , "cow" .)