Wrongdoings fashionable in the ship (4)
I believe the answer is:
sins
'wrongdoings' is the definition.
(sin is a kind of wrongdoing)
'fashionable in the ship' is the wordplay.
'fashionable' becomes 'in' (in can mean fashionable or trendy).
'in the' means one lot of letters goes inside another.
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'in' placed into 'ss' is 'SINS'.
(Other definitions for sins that I've seen before include "Moral lapses" , "Transgresses" , "Evils" , "Wrongdoings" , "Serious offences" .)