Like youth with mouth, it must be seen and not heard (3,5)
I believe the answer is:
eye rhyme
'like youth' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for eye rhyme that I've seen before include "Similarity in spelling but not sound as in love and move" , "Maid said, for example" , "shy?" , "scan piece of poetry" , "For example know how" .)