"Could be better” (3)
'could be better' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for meh that I've seen before include "Not much cop" , "Unexciting (modern slang)" , "Who cares" , "am I bovvered?" , "expression of indifference" .)
(Other definitions for not bad that I've seen before include "Reasonable" , "Middling (so good?)" , "fair" , "fine" , "could be OK" .)