Strange story about English shellfish (6)
I believe the answer is:
oyster
'shellfish' is the definition.
(I know that oyster is a type of shellfish)
'strange story about english' is the wordplay.
'strange' indicates anagramming the letters.
'about' means one lot of letters goes inside another.
'english' becomes 'E' (abbreviation).
'story' anagrammed gives 'oystr'.
'oystr' enclosing 'e' is 'OYSTER'.
(Other definitions for oyster that I've seen before include "Edible bivalve mollusc" , "Native -- seafood" , "Maybe grey" , "Pearl-producing bivalve" , "Bivalve shellfish" .)