Two fools in murderous combination (8)

I believe the answer is:

assassin

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'murderous combination' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

'two fools in' is the wordplay.
'two fools' can mean two substitutions for 'fool'.
'fool' becomes 'ass' (both can mean a stupid person).
'fool' becomes 'ass' (both can mean a stupid person).
'ass'+'ass'+'in'='ASSASSIN'

Can you help me to learn more?

(Other definitions for assassin that I've seen before include "Political murderer" , "Bravo!" , "Hired murderer" , "One murders for political or monetary reasons" , "Liquidator" .)

I've seen this clue in The Globe and Mail.

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