A chief of police or regular day-tripper? (8)

I believe the answer is:

acidhead

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

I cannot understand how the remainder of the clue works.

(Other definitions for acidhead that I've seen before include "drug user!" , "LSD user" , "tripper" , "Drug addict" .)

I've seen this clue in The Times and The Sunday Times.

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