Minute hands, essentially (5)
I believe the answer is:
radii
'minute hands essentially' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for radii that I've seen before include "Two making up diameter" , "Halves of diameter" , "Lines from centre to perimeter of a circle" , "Lines from the centre to the circumference of a circle" , "Veins of insect's wings" .)