Press sarcasm's not unknown (4)
I believe the answer is:
iron
'press' is the definition.
(ironing is a kind of pressing)
'sarcasm's not unknown' is the wordplay.
'sarcasm' becomes 'irony' ('irony' can be a synonym of 'sarcasm').
'not' indicates named letters should be taken away.
'unknown' becomes 'y' (algebra).
'irony' with 'y' taken out is 'IRON'.
(Other definitions for iron that I've seen before include "used in the evening" , "Remove creases from" , "Metal - press" , "Metal (that may be pumped?)" , "Fe - golf club" .)