When nothing is restricting, as one’s slaughtered? (4,6)

I believe the answer is:

open season

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'when' is the definition.
I know nothing about this answer so I can't tell whether this works.

'nothing is restricting as one's slaughtered?' is the wordplay.
'nothing' becomes 'O' (looks like zero - 0).
'is restricting' becomes 'pens' (pen can mean to restrict or enclose).
'slaughtered?' indicates anagramming the letters (I've seen 'slaughtered' mean this).
'as'+'one'='asone'
'asone' anagrammed gives 'eason'.
'o'+'pens'+'eason'='OPEN SEASON'

Can you help me to learn more?

(Other definitions for open season that I've seen before include "When game shootable" , "period designated for shooting" , "enjoyed by some sportsmen" , "anyone could be opposition here" , "When killing is permitted" .)

I've seen this clue in The Times.

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