Mollusc primarily encountered in fanciful story (6)
I believe the answer is:
oyster
'mollusc' is the definition.
(I know that oyster is a type of bivalve)
'primarily encountered in fanciful story' is the wordplay.
'primarily' says to take the initial letters.
'in' is an insertion indicator.
'fanciful' indicates an anagram (a fanciful or silly ordering of the letters).
The first letter of 'encountered' is 'e'.
'story' anagrammed gives 'oystr'.
'e' put into 'oystr' is 'OYSTER'.
(Other definitions for oyster that I've seen before include "Content of stew" , "Succulent shellfish" , "Pearl-producing bivalve" , "bit of chicken meat" , "taken in by carpenter" .)