It's not quite true politicians are winners! (6)
I believe the answer is:
trumps
'winners' is the definition.
Although both the answer and definition are plural nouns, I can't see how they can define each other.
'it's not quite true politicians' is the wordplay.
'it's not quite' means to remove the last letter (most but not all of the word).
'politicians' becomes 'MPs' (Members of Parliament).
'true' with its final letter taken away is 'tru'.
'tru'+'mps'='TRUMPS'
'are' is the link.
Can you help me to learn more?
(Other definitions for trumps that I've seen before include "Admired person's" , "Playing cards given added value in some games" , "Playing cards temporarily ranking above others" , "Highest-ranking suit" , "Gems" .)