Behind Cockney Trotter’s back (7)

I believe the answer is:

endorse

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'back' is the definition.
(back can mean to support or endorse)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for endorse that I've seen before include "'Approve of, back (7)'" , "No reeds (anag)" , "Champion" , "Corroborate" , "Publicly approve" .)

I've seen this clue in The Sun.

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