Navigation aid called, could be part of the last course (6)

I believe the answer is:

mousse

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'course' is the definition.
(mousse is a kind of course)

I don't understand the remainder of the clue.

(Other definitions for mousse that I've seen before include "Whipped sweet" , "Capital preparation" , "Frothy dish - hairdressing" , "Creamy dessert; hair dressing" , "hair product" .)

I've seen this clue in The Hindu.

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