Republican's second in command, please reply (4)
I believe the answer is:
rsvp
'please reply' is the definition.
(I've seen this before)
'republican's second in command' is the wordplay.
'republican's' becomes 'rs' (abbreviation).
'second in command' becomes 'vp' (short for vice president).
'rs'+'vp'='RSVP'
(Other definitions for rsvp that I've seen before include "Answer the invitation briefly, please" , "Please reply to the French, in short" , "Reply to the invitation, please" , "Answer! (Fr.)" , "Reply to invitation initially" .)