Vision you once backed with energy (3)
I believe the answer is:
eye
'vision' is the definition.
(thesaurus)
'you once backed with energy' is the wordplay.
'you once' becomes 'ye' (I've seen this in other clues**).
'backed' shows that the letters should be reversed in order.
'with' is a charade indicator (letters next to each other).
'energy' becomes 'e' (physics symbol as in E=mce).
'ye' reversed gives 'ey'.
'ey'+'e'='EYE'
(Other definitions for eye that I've seen before include "study" , "1 across, 4 across, for example" , "Glance" , "Ireland's . . ., off the Dublin coast" , "Viewer" .)