On a ship, comprehensive direction-finder (7)
I believe the answer is:
compass
'direction-finder' is the definition.
'on a ship comprehensive' is the wordplay.
'on' is a charade indicator (letters next to each other).
'ship' becomes 'ss'.
'comprehensive' becomes 'comp' (short for comprehensive school).
'a'+'ss'='ass'
'ass' put after 'comp' is 'COMPASS'.
(Other definitions for compass that I've seen before include "Device with magnetic needle for determining direction" , "Course guide" , "Range, scope" , "may be boxed" , "Directional instrument" .)