Sarcasm in press? Unknown (5)
I believe the answer is:
irony
'sarcasm' is the definition.
(I know that sarcasm can be written as irony)
'press? unknown' is the wordplay.
'press?' becomes 'iron' (ironing is a kind of pressing).
'unknown' becomes 'y' (algebra).
'iron'+'y'='IRONY'
'in' acts as a link.
(Other definitions for irony that I've seen before include "Kind of sarcasm, sounds metallic" , "Mild sarcasm" , "type of discussion, originally" , "Might this be dramatic" , "Use of words to convey the opposite of what they normally mean" .)