Not all separate (5)

I believe the answer is:

apart

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'not all separate' is the definition.
'apart' can be an answer for 'separate' (similar in meaning). I am not sure about the 'not all' bit.

'all separate' is the wordplay.
'all' becomes 'a' (I am not sure about this - if you are sure you should believe this answer much more).
'separate' becomes 'part' (both can mean to split or break apart).
'a'+'part'='APART'

Can you help me to learn more?

(Other definitions for apart that I've seen before include "Isolated" , "in isolation" , "divorced" , "Asunder" , "Separated physically" .)

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

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