Mild sarcasm (5)
I believe the answer is:
irony
'mild sarcasm' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for irony that I've seen before include "Satire, sarcasm" , "the reverse of what you'd expect" , "Mild sarcasm" , "like a metallic element" , "The use of words which is the opposite of what they say" .)