"I'm not impressed" (3)
I believe the answer is:
'i'm not impressed' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for meh that I've seen before include "expression of indifference" , "Unexciting (modern slang)" , "Not much cop" , "am I bovvered?" , "Who cares" .)