Light science work by jerks (6)
I believe the answer is:
optics
'light science' is the definition.
(science of light)
'work by jerks' is the wordplay.
'work' becomes 'op' (abbreviation for opus).
'by jerks' becomes 'tics' (I can't justify this - if you can you should give a lot more credence to this answer).
'op'+'tics'='OPTICS'
(Other definitions for optics that I've seen before include "Science of light" , "Spirit measures" , "Science of vision/light" , "Scientific studies of light and vision, the eyes" , "The scientific study of light and vision" .)