Tamarins, playful creatures not of this world (8)
I believe the answer is:
martians
'creatures not of this world' is the definition.
'tamarins playful' is the wordplay.
'playful' indicates anagramming the letters.
'tamarins' with letters rearranged gives 'MARTIANS'.
(Other definitions for martians that I've seen before include "Extraterrestrial beings" , "Aliens from the Red Planet, common in sci-fi" , "Sci-fi aliens from the war planet" , "War of the Worlds invaders" , "Little green men?" .)