cryptic clue

[rebcs]

what is a jolly unit of heat?

[JoolsTwo]

How many letters?

[kettledrum]

Therm - the RM (Royal Marine). Jolly is a slang word for Royal Marine

[starmack]

Fabulous answer Kettledrum and the right one, I think. Had been checking out other language meanings for the world "joule"

[rebcs]

thanks for your rapid response, you'll have to forgive my vagueness with the question, it's my first attempt on this site

[kettledrum]

rebcs - Welcome to the site: we're delighted to be able to help.
A few tips for future reference:
(i) It's always a good idea to give the clue exactly as it is written, as well as its number (3a / 7d etc )
(ii)Let us know how many letters are in the answer and which ones you've already got.
& (iii) Give the name of the publication / crossword as the heading.

eg: Daily Scandal Rag - Saturday / 12345 (or whatever)

6a. The jolly unit of heat. (5) T - E - M or ? H ? R ? (Please don't use dots......our old eyes have trouble counting them!)

Happy Crosswording!