Deadlock writer’s no longer fashionable (7)
I believe the answer is:
impasse
'deadlock' is the definition.
(synonyms)
'writer's no longer fashionable' is the wordplay.
'writer's' becomes 'I'm' ('I am' referring to the writer of the clue).
'no longer fashionable' becomes 'passe' (I've seen this in another clue).
'im'+'passe'='IMPASSE'
(Other definitions for impasse that I've seen before include "Lack of movement" , "cul-de-sac" , "'Deadlock, stalemate (7)'" , "deadlock results" , "Situation in which progress is impossible" .)