Male aggressors concealing American hackers (9)
I believe the answer is:
tomahawks
'hackers' is the definition.
Although both the answer and definition are plural nouns, I don't see how one could define the other.
'male aggressors concealing american' is the wordplay.
'male' becomes 'tom' (tom can mean a male cat).
'aggressors' becomes 'hawks' (I am not sure about this - if you are sure you should believe this answer much more).
'concealing' is an insertion indicator.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'tom'+'hawks'='tomhawks'
'tomhawks' going around 'a' is 'TOMAHAWKS'.
Can you help me to learn more?
(Other definitions for tomahawks that I've seen before include "Weapons used by North American Indians" , "American Indian weapons" .)