One can see through it in a number of ways (4)
I believe the answer is:
lens
'one can see' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'it in a number of ways' is the wordplay.
'it' becomes 'e' ('e' can mean 'electronic' which is similar to 'IT').
'in' indicates putting letters inside.
'a number' becomes 'L' (fifty in Roman numerals).
'of ways' becomes 'ns' (I can't justify this - if you can you should give a lot more credence to this answer).
'l'+'ns'='lns'
'e' placed into 'lns' is 'LENS'.
'through' acts as a link.
Can you help me to learn more?
(Other definitions for lens that I've seen before include "One focusing light" , "Dark cloud; shining light" , "French town" , "town in France" , "Transparent optical device" .)