Peculiar story about English mollusc (6)
I believe the answer is:
oyster
'mollusc' is the definition.
(I know that oyster is a type of bivalve)
'peculiar story about english' is the wordplay.
'peculiar' indicates an anagram (a peculiar or strange ordering of the letters).
'about' means one lot of letters goes inside another.
'english' becomes 'E' (abbreviation).
'story' anagrammed gives 'oystr'.
'oystr' placed around 'e' is 'OYSTER'.
(Other definitions for oyster that I've seen before include "Greyish pink" , "very uncommunicative person" , "offering unlimited possibilities?" , "aphrodisiac?" , "Shade of greyish white" .)