Sound off about Republican, one living abroad (10)
I believe the answer is:
expatriate
'one living abroad' is the definition.
(someone living abroad)
'sound off about republican' is the wordplay.
'sound off' becomes 'expatiate' (to expatiate is to speak or write in detail about).
'about' means one lot of letters goes inside another.
'republican' becomes 'r' (abbreviation).
'expatiate' going around 'r' is 'EXPATRIATE'.
(Other definitions for expatriate that I've seen before include "resident abroad" , "foreigner?" , "Brit abroad?" , "one based abroad?" , "in exile" .)