"Beats me" (6)
'beats me' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for dunno that I've seen before include "I'm in the dark" , "I can't help" , "It beats me" , "I don't know (informal)" , "search me" .)
(Another definition for noidea that I've seen is " 11 [I HAVENT A CLUE]".)