Tree-dweller - oik - £500 (6)

I believe the answer is:

monkey

(Other definitions for monkey that I've seen before include "Medium-sized primate" , "Any of various long-tailed primates" , "Type of wrench for a monkey" , "Long-tailed primate" , "Five hundred pounds (colloq.); imp" .)

I've seen this clue in The Guardian.

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