4 down
17
21
brief,
17
21
to
21?
(5)
17 21 brief, 17 21 to 21? (5)
hints
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checks
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hints
The whole clue is the definition (& lit or double definition).
How the clue works
The whole clue THUMB THING brief, THUMB THING to THING? is a cryptic definition mimicking a lisp, where "TH" replaces "S" throughout — read normally it says "Something brief, something to sing?" i.e. a SONG, which with the lisp becomes THONG, and a thong is indeed something brief (a skimpy garment).