Work out contract to replace Republican with Democrat (6)
I believe the answer is:
deduce
'work out' is the definition.
(to deduce is to work out or figure out)
'republican with democrat' is the wordplay.
'republican' becomes 'deuce' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'with' is an insertion indicator.
'democrat' becomes 'd'.
'deuce' placed around 'd' is 'DEDUCE'.
'contract to replace' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for deduce that I've seen before include "Conclude by reasoning" , "Reason out (something)" , "Figure out as Sherlock did" , "Infer like Sherlock" , "Arrive at conclusion through reasoning, as Sherlock Holmes did" .)