Is necessarily to be in phase (3)
I believe the answer is:
Here is my best explanation:
'is necessarily to' is the definition.
I can't judge whether this defines the answer.
'in phase' is the wordplay.
'in' says the answer is hidden in the clue.
'HAS' is hidden within 'phase'.
'be' is the link.
(Other definitions for has that I've seen before include "Enjoys", "Eats", "Cheats", "Owns, holds", "Bears", "Contains", "suffers from", "Possesses or owns" and "Gives birth to".)