Bit of fun at the saloon, when it's 'open' (5)

I believe the answer is:

unbar

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'it's open' is the definition.
(unbarring is a kind of opening)

'bit of fun at the saloon' is the wordplay.
'bit of fun at' becomes 'un' (I can't justify this - if you can you should give a lot more credence to this answer).
'the saloon' becomes 'bar' (saloon is a type of bar).
'un'+'bar'='UNBAR'

'when' is the link.

Can you help me to learn more?

(Another definition for unbar that I've seen is " Free".)

I've seen this clue in the Evening Standard.

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