United over unknown god (5)
I believe the answer is:
deity
'god' is the definition.
('deity' can be a synonym of 'god')
'united over unknown' is the wordplay.
'united' becomes 'tied' ('tied' can be similar in meaning to 'united').
'over' shows that the letters should be reversed in order.
'unknown' becomes 'y' (algebra).
'tied' reversed gives 'deit'.
'deit'+'y'='DEITY'
(Other definitions for deity that I've seen before include "Supreme being" , "Divinity" , "Supernatural being" , "God or goddess" , "Refuge" .)