It stimulates members of a government, with Republican breaking through (6)
I believe the answer is:
viagra
'it stimulates members' is the definition.
(drug improving erectile function)
'a government with republican breaking through' is the wordplay.
'government' becomes 'g' (abbreviation for government).
'with' is a charade indicator (letters next to each other).
'republican' becomes 'r' (abbreviation).
'breaking' indicates putting letters inside.
'through' becomes 'via' (via means going through).
'a'+'g'+'r'='agr'
'agr' placed within 'via' is 'VIAGRA'.
'of' acts as a link.
(Other definitions for viagra that I've seen before include "Performance-enhancing drug?" , "Drug treating impotence" , "Drug aiding one's sex life" , "Promoter of the other" , "Controversial pills" .)