Opposite of o'er (5)
I believe the answer is:
neath
'opposite of o'er' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for neath that I've seen before include "Under (poetic)" , "Under, poetically" , "Town NE of Swansea" , "Welsh rugby club" , "town in South Wales" .)