5 down
The
Royal
Marines:
the
ultimate
in
peerless
British
units
(6)
The Royal Marines: the ultimate in peerless British units (6)
hints
0
checks
0
hints
The definition is at the end of the clue.
How the clue works
Look at British units — that's your definition, since therms are units of heat energy. You get there by taking The (THE), adding Royal Marines (RM), then pinching the last letter of peerless (S) to finish it off.