Forbidding nun to keep fashionable (8)
I believe the answer is:
sinister
'forbidding' is the definition.
('sinister' can be a synonym of 'forbidding')
'nun to keep fashionable' is the wordplay.
'nun' becomes 'sister' (Sister is an example).
'to keep' means one lot of letters goes inside another.
'fashionable' becomes 'in' ('in' can be similar in meaning to 'fashionable').
'sister' placed around 'in' is 'SINISTER'.
(Other definitions for sinister that I've seen before include "Very unpleasant" , "Suggestive of evil, being lefthanded" , "can be injurious" , "Disturbing" , "Suggesting or threatening evil or danger" .)