Boy's terrifying brief encounter with a mollusc (6)
I believe the answer is:
oyster
'mollusc' is the definition.
(oyster is a kind of mollusc)
'boy's terrifying brief encounter' is the wordplay.
'OYSTER' is hidden within the letters.
But, I'm not sure how this is indicated.
'with a' acts as a link.
Can you help me to learn more?
(Other definitions for oyster that I've seen before include "Pearl-bearing sea creature" , "Shellfish - colour" , "Pearl producer" , "offering unlimited possibilities?" , "Native -- seafood" .)