Select one about line of sight (7)
I believe the answer is:
optical
'of sight' is the definition.
(I've seen this before)
'select one about line' is the wordplay.
'select' becomes 'opt' (I've seen this before).
'one' becomes 'i' (Roman numeral).
'about line' becomes 'cal' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'opt'+'i'+'cal'='OPTICAL'
Can you help me to learn more?
(Other definitions for optical that I've seen before include "some kind of illusion" , "Capitol (anag.)" , "Relating to light/vision" , "Topical (anag.)" , "Involving light" .)