Recoil at sea catching fish, snapper? (9)
I believe the answer is:
crocodile
'snapper?' is the definition.
(an animal which 'snaps')
'recoil at sea catching fish' is the wordplay.
'at sea' is an anagram indicator.
'catching' indicates putting letters inside.
'fish' becomes 'cod' (type of fish).
'recoil' with letters rearranged gives 'croile'.
'croile' enclosing 'cod' is 'CROCODILE'.
(Other definitions for crocodile that I've seen before include "Line of children walking in twos" , "line of schoolkids?" , "Insincere show of sorrow" , "Vicious beast" , "Large reptile with long jaws" .)