Foreign allies love punching American fighters (6)
I believe the answer is:
amigos
'foreign allies' is the definition.
The definition suggests a plural noun which matches the answer.
'love punching american fighters' is the wordplay.
'love' becomes 'o' (love means zero in tennis).
'punching' means one lot of letters goes inside another (inserted letters punch or puncture others).
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'fighters' becomes 'migs' (I am not sure about this - if you are sure you should believe this answer much more).
'a'+'migs'='amigs'
'o' inserted into 'amigs' is 'AMIGOS'.
Can you help me to learn more?
(Other definitions for amigos that I've seen before include "chums" , "comrades in Spain" , "Friends (Spanish)" , "Friends in Spain" , "Friends (Span.)" .)