Friend giving a Russian fighter oxygen (5)
I believe the answer is:
amigo
'friend' is the definition.
(thesaurus)
'giving a russian fighter oxygen' is the wordplay.
'giving' means one lot of letters go next to another.
'russian fighter' becomes 'mig' (Russian fighter plane manufacturer).
'oxygen' becomes 'o' (atomic symbol for oxygen).
'a' next to 'mig' is 'amig'.
'amig'+'o'='AMIGO'
(Other definitions for amigo that I've seen before include "Mexican's mate" , "A Spanish pal" , "Pal from Madrid?" , "Hispanic friend" , "Mate in Madrid?" .)