Press unknown to show sarcasm (5)
I believe the answer is:
irony
'sarcasm' is the definition.
('irony' can be a synonym of 'sarcasm')
'press unknown' is the wordplay.
'press' becomes 'iron' (ironing is a kind of pressing).
'unknown' becomes 'y' (algebra).
'iron'+'y'='IRONY'
'to show' acts as a link.
(Other definitions for irony that I've seen before include "style of humour" , "the reverse of what you'd expect" , "Kind of sarcasm, sounds metallic" , "Scornful wit" , "situation showing incongruity" .)