DUNNO

No idea (informal) (5)

Solution has appeared in 3 crosswords (1 across and 2 down):

• Quick crossword No 16,018 — 2021-09-07 23:00:21
  11A: I've no idea (informal) (5)
• Quick crossword No 15,827 — 2021-01-28 00:00:00
  21D: Haven't a clue! (informal) (5)
• Quick crossword No 15,455 — 2019-11-19 00:00:48
  16D: No idea (informal) (5)