Sold out by a politician you've often heard that before (3,5)
I believe the answer is:
old story
'you've often heard that before' is the definition.
I don't know anything about this answer so I can't judge whether this works.
'sold out by a politician' is the wordplay.
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'by' means one lot of letters go next to another.
'a politician' becomes 'tory' (I've seen this before).
'sold' anagrammed gives 'olds'.
'olds'+'tory'='OLD STORY'
Can you help me to learn more?
(Another definition for old story that I've seen is " Familiar tale".)