One kills when best troops go astray (8)

I believe the answer is:

assassin

I'm a little stuck... Click here to teach me more about this clue!

'one kills' is the definition.

'when best troops go astray' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'best troops' becomes 'sas' (special air service. I am not sure about the 'best' bit.).
'go astray' becomes 'sin' (I've seen this in another clue).
'as'+'sas'+'sin'='ASSASSIN'

Can you help me to learn more?

(Other definitions for assassin that I've seen before include "Hired killer" , "Paid killer" , "Professional, often political, killer" , "Political murderer" , "One murders for political or monetary reasons" .)

I've seen this clue in The Sun.

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