Dumb loyalty regularly needed to serve on such a vessel (1-4)
I believe the answer is:
u-boat
'to serve on such a vessel' is the definition.
(German submarine in WW1 and WW2)
'dumb loyalty regularly' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'dumb'+'loyalty'='dumbloyalty'
The alternate letters of 'dumbloyalty' are 'U-BOAT'.
'needed' acts as a link.
(Other definitions for u-boat that I've seen before include "Sea vessel" , "Submersible warship of WWII" , "Submersible warship armed with torpedoes" , "German wartime submarine" , "Take pool exercise" .)