Sarcasm unknown to press at first (5)
I believe the answer is:
irony
'sarcasm' is the definition.
(I know that sarcasm can be written as irony)
'unknown to press at first' is the wordplay.
'unknown' becomes 'y' (algebra).
'to press' becomes 'iron' (ironing is a kind of pressing).
'at first' means one lot of letters go next to another.
'y' after 'iron' is 'IRONY'.
(Other definitions for irony that I've seen before include "Conveying meaning by expressing the opposite" , "Form of satire, like Fe" , "form of sarcasm" , "Gentle sarcasm" , "Figure of speech" .)