Mockery is hard on a youth leader (5)
I believe the answer is:
irony
'mockery' is the definition.
(I've seen this before)
'hard on a youth leader' is the wordplay.
'hard' becomes 'ir' (I can't explain this - if you can you should give a lot more credence to this answer).
'a youth leader' becomes 'y' (I've seen this in other clues).
'ir'+'on'+'y'='IRONY'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for irony that I've seen before include "Caustic remark" , "Use of words to imply the opposite of what they mean" , "type of discussion, originally" , "Gentle sarcasm" , "like a metallic element" .)