Caustic press in charge (6)
I believe the answer is:
ironic
'caustic' is the definition.
(thesaurus)
'press in charge' is the wordplay.
'press' becomes 'iron' (ironing is a kind of pressing).
'in charge' becomes 'ic'.
'iron'+'ic'='IRONIC'
(Other definitions for ironic that I've seen before include "Paradoxical, contradictory" , "Being sarcastic or deliberately oblique" , "A form of sarcasm, may sound perverse" , "Sarcastic or deliberately perverse" , "turning out unexpectedly" .)