Planet inhabited by minute alien creature (8)
I believe the answer is:
marmoset
'creature' is the definition.
(I've seen this before)
'planet inhabited by minute alien' is the wordplay.
'planet' becomes 'mars' (Mars is a planet).
'inhabited by' means one lot of letters goes inside another.
'minute' becomes 'mo' (both can mean a short amount of time).
'alien' becomes 'et' (extraterrestrial).
'mars' placed around 'mo' is 'marmos'.
'marmos'+'et'='MARMOSET'
(Other definitions for marmoset that I've seen before include "Small tropical American monkey" , "Monkey with claws" , "Small New World monkey" , "Somme rat? (anag)" , "Star memo about kind of monkey" .)