Press agency’s ultimate paradox (5)
I believe the answer is:
irony
'paradox' is the definition.
(thesaurus)
'press agency's ultimate' is the wordplay.
'press' becomes 'iron' (ironing is a kind of pressing).
'ultimate' indicates one should take the final letters.
The final letter of 'agency' is 'y'.
'iron'+'y'='IRONY'
(Other definitions for irony that I've seen before include "Sarcastic humour" , "What "as clear as mud" demonstrates" , "Socratic method of discussion" , "Mild sarcasm" , "situation showing incongruity" .)