First murderer takes in clever Macheath say (7)

I believe the answer is:

captain

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'macheath say' is the definition.
I can't tell whether this defines the answer.

'first murderer takes in clever' is the wordplay.
'first murderer' becomes 'cain' (I've seen this before).
'takes in' is an insertion indicator.
'clever' becomes 'apt' (synonyms).
'cain' going around 'apt' is 'CAPTAIN'.

Can you help me to learn more?

(Other definitions for captain that I've seen before include "Skipper" , "soldier" , "Lead (a team)" , "Ship's commander" , "One traditionally goes down with a sinking ship" .)

I've seen this clue in The Independent.

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