A Republican work mentioned rising migration (8)
I believe the answer is:
diaspora
'migration' is the definition.
(I've seen this in another clue)
'a republican work mentioned rising' is the wordplay.
'republican' becomes 'R' (abbreviation).
'work' becomes 'op' (abbreviation for opus).
'mentioned' becomes 'said' (I've seen this before).
'rising' shows that the letters should be reversed in order (in a down clue, letters go up).
'a'+'r'+'op'+'said'='aropsaid'
'aropsaid' backwards is 'DIASPORA'.
(Other definitions for diaspora that I've seen before include "The dispersal of a people" , "Dispersion of a people from their homeland" , "People scattered" , "The dispersion of Jews throughout the world" , "A great scattering, as of Jews from Egypt" .)