Permit naive Republican to get taken in by rotter (5,4)
I believe the answer is:
green card
'permit' is the definition.
(green card is a type of pass or permit)
'naive republican to get taken in by rotter' is the wordplay.
'naive' becomes 'green' (similar in meaning).
'republican' becomes 'R' (political abbreviation).
'to get' is an insertion indicator.
'taken in by rotter' becomes 'cad' (I can't explain this - if you can you should give a lot more credence to this answer).
'r' put into 'cad' is 'card'.
'green'+'card'='GREEN CARD'
Can you help me to learn more?
(Another definition for green card that I've seen is " US work and residence permit for foreign nationals".)