Navigational aid, no end useful to Texans (7)
I believe the answer is:
sextant
'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" .)