Eye relative up (3)
I believe the answer is:
orb
'eye' is the definition.
(thesaurus)
'relative up' is the wordplay.
'relative' becomes 'bro' (I've seen this before).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'bro' in reverse letter order is 'ORB'.
(Other definitions for orb that I've seen before include "Sovereign artefact" , "Ornamental sphere" , "Sphere for a sovereign" , "Ring" , "component of royal regalia" .)