Upset ransacker to re-equip workshop? (6)

I believe the answer is:

retool

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'upset ransacker to re-equip workshop?' is the definition.
I can't judge whether this definition defines the answer.

'upset ransacker to re' is the wordplay.
'upset ransacker' becomes 'tool' (I can't explain this - if you can you should believe this answer much more).
'to' means one lot of letters go next to another (I've seen this in other clues).
'tool' after 're' is 'RETOOL'.

Can you help me to learn more?

(Another definition for retool that I've seen is " Rearrange equipment".)

I've seen this clue in The Telegraph.

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