Tool on a chain, perhaps (3)

I believe the answer is:

saw

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'tool' is the definition.
(saw is a kind of tool)

'on a chain perhaps' is the wordplay.
'on' means one lot of letters goes inside another (as in clothing 'on' a person).
'chain perhaps' becomes 'sw' (I can't explain this - if you can you should believe this answer much more).
'a' placed inside 'sw' is 'SAW'.

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(Other definitions for saw that I've seen before include "Viewed; adage" , "Gnome" , "Perceived visually" , "Wise saying or cutting tool" , "Noticed - tool" .)

I've seen this clue in the Evening Standard.

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