Marine creature, small, this far out (8)
I believe the answer is:
starfish
'marine creature' is the definition.
'small this far out' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'this'+'far'='thisfar'
'thisfar' anagrammed gives 'tarfish'.
's'+'tarfish'='STARFISH'
(Other definitions for starfish that I've seen before include "Sea creature with five arms coming from central disk" , "Has rifts (anag)" , "Five-armed sea creature" , "Marine creature with five arms" , "Asteroid (in the sea)" .)