Like heavy metal? That’s mild sarcasm (5)
I believe the answer is:
irony
'mild sarcasm' is the definition.
(I've seen this before)
'like heavy metal?' is the wordplay.
I cannot quite understand how this works, but
'like' could be 'y' (similar in meaning) and 'y' is found within the answer.
'metal?' could be 'ir' (Ir is an example) and 'ir' is found in the answer.
The remaining letters 'on' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'that's' is the link.
Can you help me to learn more?
(Other definitions for irony that I've seen before include "Mild sarcasm" , "Satirical speech form" , "The use of words which is the opposite of what they say" , "like a metallic element" , "form of sarcasm" .)