Trouble when unruly gang sent back head boy with item bought in joke shop (9)

I believe the answer is:

stinkbomb

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'unruly gang sent back head boy with item bought in joke shop' is the definition.
I don't know anything about this answer so I can't tell whether it can be defined by this definition.

'trouble when' is the wordplay.
'trouble' becomes 'stink' (I've seen this before).
'when' becomes 'bomb' (I can't explain this - if you can you should believe this answer much more).
'stink'+'bomb'='STINKBOMB'

Can you help me to learn more?

I've seen this clue in The Irish Times.

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