Two sets of one hundred thousand anti-aircraft weaponry (3-3)
I believe the answer is:
ack-ack
'weaponry' is the definition.
(ack-ack is a kind of weaponry)
'two sets of one hundred thousand anti-aircraft' is the wordplay.
'two' says to double the letters.
'sets of' is an anagram indicator (I've seen 'setting' mean this).
'one hundred' becomes 'c' (Roman numeral).
'thousand' becomes 'a' (synonyms).
'anti aircraft' becomes 'k' (I can't justify this - if you can you should believe this answer much more).
'c'+'a'='ca'
'ca' anagrammed gives 'ac'.
'ac'+'k'='ack'
'ack' duplicated is 'ACK-ACK'.
Can you help me to learn more?
(Other definitions for ack-ack that I've seen before include "Anti-aircraft fire" , "Anti-aircraft gun" , "Anti-aircraft gunfire" .)