Boy's terrified catching sea creature (6)
I believe the answer is:
oyster
'sea creature' is the definition.
(I've seen this before)
'boy's terrified catching' is the wordplay.
'catching' indicates the answer is hidden within the clue.
'OYSTER' is hidden in the letters of 'boys terrified'.
(Other definitions for oyster that I've seen before include "Shellfish (maybe the world!)" , "It produces pearls" , "offers all one could desire" , "Bivalve native" , "Seafood item sometimes eaten alive" .)