Unfavourable notice on output of poet (7)
I believe the answer is:
adverse
'unfavourable' is the definition.
(similar in meaning)
'notice on output of poet' is the wordplay.
'notice' becomes 'ad' (short for advert).
'on' is a charade indicator (letters next to each other).
'output of poet' becomes 'verse' (a poet writes verse).
'ad'+'verse'='ADVERSE'
(Other definitions for adverse that I've seen before include "In an opposing direction" , "Opposed - inauspicious - opposite" , "Hostile" , "'Contrary, untoward (7)'" , "Dire" .)