Are you sure about BESOM, AJT? I've only ever seen it as a broom made of twigs, and I can't find any usage relating to the clue. Is it a Scottish slang word? Are those letters absolutely certain Bryan?
The letters i have for is are correct (B?S?M) ithink besom is right because 'cheeky' seems to tie in with it.
83A Musical note (3) S?H The S comes from INSTANCE (occasion) and H from CHILDISH (immature)
I looked at the glossary of musical terms in wiki but nothing fits.