Crossword Genius

Dishevelled rogues join Conservative for a laugh (6-3,5)

Ross

I believe the answer is:

shaggy-dog story

I'm a little stuck... Click here to teach me more about this clue!

'laugh' is the definition.
(shaggy dog story is a kind of laugh)

'dishevelled rogues join conservative for a' is the wordplay.
I cannot really understand how this works, but
'conservative' could be 'tory' (informal term for a Conservative Party member) and 'tory' is present in the answer.
'a' is present in the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

I've seen this clue in The Independent.
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