Comeback from Republican that is holding office (7)
I believe the answer is:
riposte
'comeback' is the definition.
('riposte' can be a synonym of 'comeback')
'republican that is holding office' is the wordplay.
'republican' becomes 'r' (abbreviation).
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'holding' indicates putting letters inside.
'office' becomes 'post' ('post' can be a synonym of 'office').
'ie' placed around 'post' is 'iposte'.
'r'+'iposte'='RIPOSTE'
'from' acts as a link.
(Other definitions for riposte that I've seen before include "Rejoinder" , "Fencing counterattack" , "Return thrust or repartee" , "Reply, retort" , "Quick clever reply" .)