Republican group is after firm support (6)
I believe the answer is:
corset
'support' is the definition.
(thesaurus)
'republican group is after firm' is the wordplay.
'republican' becomes 'R' (abbreviation).
'group' becomes 'set' (both can mean a collection).
'is after' says to put letters next to each other.
'firm' becomes 'co' (a firm is a company).
'r'+'set'='rset'
'rset' after 'co' is 'CORSET'.
(Other definitions for corset that I've seen before include "Sector (anag)" , "female garment!" , "A lady's old foundation garment" , "Stiffened undergarment" , "Stays" .)