Does his trade involve mixing oil into tar? (6)

I believe the answer is:

tailor

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'does his trade involve' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.

'mixing oil into tar?' is the wordplay.
'mixing' indicates an anagram.
'into' means one lot of letters goes inside another.
'oil' is an anagram of 'ilo'.
'ilo' inserted within 'tar' is 'TAILOR'.

Can you help me to learn more?

(Other definitions for tailor that I've seen before include "Clothes-maker" , "Maker of clothes, men's mostly" , "One who makes garments" , "Cut" , "One makes fitted clothes" .)

I've seen this clue in the Evening Standard.

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