I had energy put into slim lamp (9)
I believe the answer is:
sidelight
'lamp' is the definition.
(sidelight is a kind of lamp)
'i had energy put into slim' is the wordplay.
'i had' becomes 'id' (I'd).
'energy' becomes 'e' (physics symbol as in E=mc2).
'put into' means one lot of letters goes inside another.
'slim' becomes 'slight' ('slight' can be a synonym of 'slim').
'id'+'e'='ide'
'ide' going into 'slight' is 'SIDELIGHT'.
(Other definitions for sidelight that I've seen before include "Additional or incidental information" , "Visible warning" , "One shines" , "thin illumination" , "An essential for car" .)