One’s not moved border up (3)
I believe the answer is:
Here is my best explanation:
'one's not moved' is the definition.
(both would let a speaker express indifference)
'border up' is the subsidiary indication.
'border' becomes 'hem' (I've seen this before).
'up' is a reversal indicator (in down clue: letters go upwards).
'hem' reversed gives 'meh'.
(Other definitions for meh that I've seen before include "Unexciting (modern slang)", "Who cares", "Not much cop", "am I bovvered?" and "expression of indifference".)