Crossword Genius

Navigational aid, no end useful to Texans (7)

Ross

I believe the answer is:

sextant

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'navigational aid' is the definition.
(I've seen this before)

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for sextant that I've seen before include "Navigational instrument for finding position" , "Measuring instrument on board" , "Sighting instrument" , "Measuring instrument for navigation" , "Astronavigation instrument" .)

I've seen this clue in the Evening Standard.
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