Remain in position (3)
I believe the answer is:
sit
'remain' is the definition.
The answer and definition can be both to do with fundamental existence as well as being verbs in their base form.
Maybe there's a link between them I don't understand?
'in position' is the wordplay.
'in' indicates a hidden word.
'SIT' is hidden within 'position'.
Can you help me to learn more?
(Other definitions for sit that I've seen before include "Pose (for artist)" , "Take a chair" , "Serve (on jury)" , "To take the test" , "Rest on one's haunches" .)