Ring about including part - one relating to the eye (5)
I believe the answer is:
optic
'relating to the eye' is the definition.
(I've seen this before)
'ring about including part one' is the wordplay.
'ring about' becomes 'oc' (I can't justify this - if you can you should believe this answer much more).
'including' means one lot of letters goes inside another.
'part' becomes 'pt' (abbreviation).
'one' becomes 'I' (Roman numeral).
'pt'+'i'='pti'
'oc' enclosing 'pti' is 'OPTIC'.
Can you help me to learn more?
(Other definitions for optic that I've seen before include "Picot (anag.)" , "Of sight or the eye" , "Related to sight" , "Device for measuring gin etc" , "affecting vision?" .)