Excellent proposal to restrain Republican (5)
I believe the answer is:
brill
'excellent' is the definition.
(I've seen this before)
'proposal to restrain republican' is the wordplay.
'proposal' becomes 'bill' (I have seen 'parliamentary proposal' mean 'bill' so perhaps 'proposal' could also mean 'bill').
'to restrain' is an insertion indicator.
'republican' becomes 'r' (abbreviation).
'bill' enclosing 'r' is 'BRILL'.
(Other definitions for brill that I've seen before include "Great" , "swimmer" , "Very good - fish" , "Fab -- fish" , "Flatfish; excellent" .)