Long-tailed primate (6)
I believe the answer is:
monkey
'long-tailed primate' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for monkey that I've seen before include "Five hundred pounds (colloq.); imp" , "Primate with a long tail" , "Araucaria -- evergreen conifer" , "Any of various long-tailed primates" , "Primate; five hundred (slang)" .)