Pivotal; central (5)
I believe the answer is:
focal
I believe this is a double definition.
'pivotal' is the first definition.
(I've seen this in another clue)
'central' is the second definition.
(similar in meaning)
(Other definitions for focal that I've seen before include "-- Length, -- plane; -- point" , "Relating to a centre of interest" , "Type of length or point" , "Of the point of convergence" , "- length characterises a lens" .)