Officer material, ace joining Nato oddly (8)
I believe the answer is:
sergeant
'officer' is the definition.
(sergeant is a kind of officer)
'material ace joining nato oddly' is the wordplay.
'material' becomes 'serge' (serge is a kind of material).
'ace' becomes 'a'.
'joining' says to put letters next to each other.
'oddly' indicates alternate letters.
The alternate letters of 'nato' are 'nt'.
'serge'+'a'+'nt'='SERGEANT'
(Other definitions for sergeant that I've seen before include "CARRY ON FILM" , "Estrange (anag)" , "Man in charge" , "Soldier and lover in Hardy novel" , "Copper" .)