Concentrate on what a lens does (5)
I believe the answer is:
focus
'concentrate' is the definition.
(I know that concentrate can be written as focus)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for focus that I've seen before include "Spotlight" , "Centre of interest or activity (5)" , "Bull's eye" , "Hub" , "Adjust (camera lens)" .)