Republican welcomed by gun firm (5)
I believe the answer is:
stern
'firm' is the definition.
(I've seen this in another clue)
'republican welcomed by gun' is the wordplay.
'republican' becomes 'R' (political abbreviation).
'welcomed by' means one lot of letters goes inside another (letters welcomed inside).
'gun' becomes 'sten' (British sub-machine gun).
'r' inserted into 'sten' is 'STERN'.
(Other definitions for stern that I've seen before include "enforcing submission" , "Rigid" , "Severe -- back of boat" , "Severe, demanding" , "Dour" .)