Excursionist guilty of foul play? (7)

I believe the answer is:

tripper

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'excursionist' is the definition.
(I know that excursionist can be written as tripper)

I cannot understand how the remainder of the clue works.

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(Other definitions for tripper that I've seen before include "person having highly enjoyable experience?" , "One out for pleasure" , "Person on an excursion" , "Holidaymaker" , "Someone on an excursion" .)

I've seen this clue in the Evening Standard.

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