Small arboreal ape (6)
I believe the answer is:
gibbon
'small arboreal ape' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for gibbon that I've seen before include "A historian; SE Asian forest creature" , "an eighteenth-century author" , "Member of the ape family" , "Small ape - famous historian" , "Writer" .)