Window attachments badly installed in ship (5)
I believe the answer is:
sills
'window attachments' is the definition.
(window sills)
'badly installed in ship' is the wordplay.
'badly' becomes 'i'll' ('ill' can be adverbial as in 'ill-advised').
'installed in' indicates putting letters inside.
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'ill' put into 'ss' is 'SILLS'.
(Other definitions for sills that I've seen before include "Ledges" , "Horizontal slabs under windows" .)