Changed story about English mollusc (6)
I believe the answer is:
oyster
'mollusc' is the definition.
(I know that oyster is a type of bivalve)
'changed story about english' is the wordplay.
'changed' indicates anagramming the letters.
'about' means one lot of letters goes inside another.
'english' becomes 'E' (abbreviation).
'story' is an anagram of 'oystr'.
'oystr' enclosing 'e' is 'OYSTER'.
(Other definitions for oyster that I've seen before include "Source of pearl" , "Secretive individual" , "One eaten by Walrus and Carpenter" , "Edible bivalve mollusc" , "Capital card" .)