"I'm at a loss” (5)
I believe the answer is:
dunno
'i'm at a loss' 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 "Informally, pass" , "No idea (informal)" , "I'm in the dark" , "I've no idea!" , "It beats me" .)