Prison guards work party on board (6)
I believe the answer is:
screws
'prison guards' is the definition.
('screw' can be a synonym of 'prison guard')
'work party on board' is the wordplay.
'work party' becomes 'crew' (synonyms).
'on' means one lot of letters goes inside another.
'board' becomes 'ss' (I've seen this in other clues).
'crew' inserted inside 'ss' is 'SCREWS'.
(Other definitions for screws that I've seen before include "Cheats" , "Fasteners with threaded shanks" , "Grooved type of nails" , "increase the pressure" , "Pins with spiral threads" .)