It's clear that the party leader had been lying (5)
I believe the answer is:
plain
'it's clear' is the definition.
(similar in meaning)
'the party leader had been lying' is the wordplay.
'the party leader' becomes 'p' (leader of the word 'party').
'had' says to put letters next to each other.
'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'
'that' is the link.
Can you help me to learn more?
(Other definitions for plain that I've seen before include "Unflavoured" , "Unembellished" , "open country" , "Geographical feature" , "Simply" .)