Rather broad thing their jam is served in? (6)

I believe the answer is:

widish

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'rather broad' is the definition.
(I've seen this before)

'thing their jam is served in?' is the wordplay.
I cannot really understand how this works, but
'is' is found in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Another definition for widish that I've seen is " Rather broad".)

I've seen this clue in The Times.

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