Go gaily on an excursion (4)

I believe the answer is:

trip

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

I don't understand the remainder of the clue.

(Other definitions for trip that I've seen before include "Hallucinatory experience" , "Set off for" , "Accidental misstep" , "Excursion (while on drugs?)" , "Ride" .)

I've seen this clue in the Evening Standard.

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