Pool necessity (3)
I believe the answer is:
cue
'pool necessity' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for cue that I've seen before include "Tapering stick used by snooker player" , "Used in snooker and on stage" , "Sporting rod" , "Reminder to do something" , "hit ball, in a way" .)