Chief canal nearly jammed by early arrival (7)

I believe the answer is:

supreme

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'chief' is the definition.
(I've seen this before)

I don't understand the rest of the clue.

(Other definitions for supreme that I've seen before include "Crowning" , "Most excellent" , "creamy dish of chicken perhaps" , "Creamy sauce" , "God" .)

I've seen this clue in The Times.

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