It's obvious the political leader had been lying (5)
I believe the answer is:
plain
'it's obvious' is the definition.
(similar in meaning)
'political leader had been lying' is the wordplay.
'leader' indicates taking the first letters.
'had' is a charade indicator (letters next to each other).
'been lying' becomes 'lain' (I am not sure about this - if you are sure you should believe this answer much more).
The first letter of 'political' is 'p'.
'p'+'lain'='PLAIN'
'the' is the link.
Can you help me to learn more?
(Other definitions for plain that I've seen before include "Simple, unadorned" , "Ordinary; area of land" , "Unflavoured" , "Not elaborate; clear" , "Not complicated" .)