Nato uses it as a contribution to vital phase (5)
I believe the answer is:
alpha
'nato uses it' is the definition.
I can't tell whether this defines the answer.
'a contribution to vital phase' is the wordplay.
'a contribution to' indicates a hidden word.
'ALPHA' is hidden in the letters of 'vital phase'.
'as' is the link.
Can you help me to learn more?
(Other definitions for alpha that I've seen before include "leader in Athens, originally" , "letter opener" , "letter written in Greek" , "best score" , "First in order of importance (for a Greek?)" .)