Somewhat vacuous congressman could not be trusted (6)

I believe the answer is:

conman

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'somewhat vacuous congressman' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.

'not be trusted' is the wordplay.
'not' becomes 'con' (I've seen this in other clues).
'be trusted' becomes 'man' (I can't justify this - if you can you should give a lot more credence to this answer).
'con'+'man'='CONMAN'

'could' acts as a link.

Can you help me to learn more?

(Another definition for conman that I've seen is " who's full of tricks?".)

I've seen this clue in The Irish Times.

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