Like a point of interest in optics (5)
I believe the answer is:
focal
'point of interest in optics' is the definition.
'focal' can be an answer for 'point' (focal point is a kind of point). I'm not certain of the 'of interest in optics' bit.
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for focal that I've seen before include "Things are concentrated at such a point" , "sort of length" , "Type of length or point" , "Relating to a centre of interest" , "Relating to a central point of interest" .)