Insinuates one politician isn't truthful (7)
I believe the answer is:
implies
'insinuates' is the definition.
(I've seen this before)
'one politician isn't truthful' is the wordplay.
'one' becomes 'I' (Roman numeral).
'politician' becomes 'MP' (Member of Parliament).
'isn't truthful' becomes 'lies' (I've seen this in another clue).
'i'+'mp'+'lies'='IMPLIES'
(Other definitions for implies that I've seen before include "States indirectly" , "Suggests or hints at meaning" , "Insinuates" .)