Regularly use satnav, driver's ultimate navigational aid (4)
I believe the answer is:
star
'navigational aid' is the definition.
(I've seen this in another clue)
'regularly use satnav driver's ultimate' is the wordplay.
'regularly use' means one should take alternating letters (I've seen 'regularly used' mean this (letters selected on a regular basis)).
'ultimate' suggests the final letters (the last or ultimate letter).
The last letter of 'driver' is 'r'.
The alternate letters of 'satnav' are 'sta'.
'sta'+'r'='STAR'
(Other definitions for star that I've seen before include "White blaze on a horse's forehead" , "Big name" , "appear above others?" , "huge body of gas?" , "Brilliant person" .)