A little boy's terrified by a mollusc (6)
I believe the answer is:
oyster
'a mollusc' is the definition.
(I know that oyster is a type of bivalve)
'a little boy's terrified by' is the wordplay.
'a little' indicates a hidden word (a little bit of the clue).
'OYSTER' is hidden in the letters of 'boys terrified by'.
(Other definitions for oyster that I've seen before include "Bivalve that grows pearls" , "Parent of the pearl" , "Pearl-generating bivalve" , "Capital card" , "Tasty bivalve, may carry pearl" .)