It would be a euphenmism to call it "risque" instead of "dirty."

I use euphemism to mean an acceptable substitute for an unacceptable [pick your part of speech]. "Custodial engineer" for "janitor" springs to mind, janitor being interpretable as degrading, lower-caste, shameful, undesirable, and generally negative if you are inclined to take it so. No double-meaning there at all.

"Takes a lot of brass to play the tuba" is a pun by virtue of the tuba being made of a lot of brass, but it isn't a double-entendre since there's nothing salacious about the implied meaning. Even though it does indeed have two meanings: nerve, and metal (mettle?).