Energy invested in dubious English type of lens (7)
I believe the answer is:
fisheye
'type of lens' is the definition.
'energy invested in dubious english' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'invested in' indicates putting letters inside.
'dubious' becomes 'fishy' (I've seen this before).
'english' becomes 'e' (abbreviation).
'e' put into 'fishy' is 'fishey'.
'fishey'+'e'='FISHEYE'
(Other definitions for fisheye that I've seen before include "Very wide-angle lens" , "Kind of lens" .)