It's clear the party leader had been lying (5)

I believe the answer is:

plain

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'it's clear' is the definition.
(similar in meaning)

'party leader had been lying' is the wordplay.
'party leader' becomes 'p' (leader of the word 'party').
'had' means one lot of letters go next to another.
'been lying' becomes 'lain' (I can't justify this - if you can you should give a lot more credence to this answer).
'p'+'lain'='PLAIN'

'the' acts as a link.

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(Other definitions for plain that I've seen before include "Not complicated" , "no oil painting features" , "Sheer; frank" , "Uncomplicated" , "Clear, candid" .)

I've seen this clue in the Evening Standard.

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