トウ, す.く, す.かす, す.ける, とう.る, とう.す, とおる
transparent, permeate, filter, penetrate
JLPT1 G7 S10 F1035
秀# surpass + 辵 movement → *penetrate*/*pass through* in surpassing → *create a gap/space* → *opening* (← gap) → *look through* (← look through a gap) → *transparency* (← be able to look/see through). Also, *thin* (out), *empty out* and *be free/unoccupied*, as well as *watermark* (← mark made visible by transparency).
[Full Kanji Etymology List]