They keep watch, perhaps, aboard ship (8)
I believe the answer is:
stickers
'they keep' is the definition.
(people who keep or stick at something)
'watch perhaps aboard ship' is the wordplay.
'watch perhaps' becomes 'ticker' (something which ticks).
'aboard' is an insertion indicator.
'ship' becomes 'ss'.
'ticker' put within 'ss' is 'STICKERS'.
(Other definitions for stickers that I've seen before include "Adhesive labels" , "Maybe notices" .)