NATO is one friend in a million (7)
I believe the answer is:
acronym
'nato is one' is the definition.
(NATO is an acronym)
'friend in a million' is the wordplay.
'friend' becomes 'crony' (crony is a kind of friend).
'in' indicates putting letters inside.
'million' becomes 'm' (abbreviation).
'a'+'m'='am'
'crony' put into 'am' is 'ACRONYM'.
(Other definitions for acronym that I've seen before include "eg Ernie; radar" , "Eg, Nato, Sars" , "A word initially coined" , "FBI?" , "Radar or Nato, maybe" .)