Energy's put into mounting television screen (3)
I believe the answer is:
vet
'screen' is the definition.
(thesaurus)
'energy's put into mounting television' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'put into' is an insertion indicator.
'mounting' is a reversal indicator (going up in a down clue).
'television' becomes 'tv' ('TV' can be a synonym of 'television').
'tv' back-to-front is 'vt'.
'e' placed within 'vt' is 'VET'.
(Other definitions for vet that I've seen before include "Check someone for suitability" , "Examine, check" , "Old soldier (inf.)" , "Animal doctor, in brief" , "'Examine this doctor, in short (3)'" .)