Navigation instrument (7)
I believe the answer is:
sextant
'navigation instrument' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sextant that I've seen before include "I'll tell you where sailors are" , "Measuring instrument on board" , "Nautical instrument for measuring angular distances" , "Astronavigation instrument" , "Sighting instrument" .)