Mostly employ Union to replace insurgents, initially, after an uprising in submarine (8)
I believe the answer is:
nautilus
'submarine' is the definition.
(nautilus is a kind of submarine)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for nautilus that I've seen before include "Nemo's sub (Verne)" , "craft in fiction" , "Spiral-shelled mollusc" , "Sea creature to be found in the usual tin, unusually" , "'Pearly' mollusc" .)