Bit of body art, informally

TAT

Think of the short, slangy term for what sailors might get to mark their travels.

"Bit of body art, informally" is a casual way to refer to a tattoo, which is often shortened to "tat" in everyday speech. The clue plays on the slang term rather than the formal word, making it a straightforward abbreviation puzzle. Think of how people might say "I got a new tat" instead of "I got a new tattoo."