Ok, 17D “funny cartoon about jolly diver” has to be cormorant, surely? Cartoon anag I got but that leaves RM. A jolly or jolly-boat is a tender carried by a bigger ship but does that get me RM? Royal Marine?
Well, I found the stuff about the origins of the term interesting - especially the stuff about the Trained Bands Of The City of London. I wouldn't have looked this up if it hadn't been for the query about the clue!