Swear about loud football hooligans (6)
I believe the answer is:
affirm
'swear' is the definition.
(I know that swear can be written as affirm)
'about loud football hooligans' is the wordplay.
'about' becomes 'a' (abbreviation in Chambers).
'loud' becomes 'f' (short for forte - 'loud' in music).
'football hooligans' becomes 'firm' (I can't explain this - if you can you should believe this answer much more).
'a'+'f'+'firm'='AFFIRM'
Can you help me to learn more?
(Other definitions for affirm that I've seen before include "Confidently assert" , "State as a fact, perhaps in court" , "Endorse" , "Verify" , "Assert to be the case" .)