What’s giving instrument mobile bearing aiding levelling, primarily (6)
I believe the answer is:
gimbal
'what's giving instrument mobile bearing aiding levelling primarily' is the definition.
I can't judge whether this definition defines the answer.
'giving instrument mobile bearing aiding levelling primarily' is the wordplay.
'primarily' suggests taking the first letters.
The initial letters of 'giving instrument mobile bearing aiding levelling' is 'GIMBAL'.
(Other definitions for gimbal that I've seen before include "Device that keeps something horizontal while its support tips" , "Ship's compass mount" , "Compass mounting" , "It allows a ship's compass to remain horizontal" , "Appliance to allow a ship's compass to remain horizontal" .)