Gosh! Tailless sea birds! (5)
I believe the answer is:
geese
'birds' is the definition.
(goose is a kind of bird)
'gosh tailless sea' is the wordplay.
'gosh' becomes 'gee' (I've seen this in another clue).
'tailless' means to remove the last letter (the tail of the word is removed).
'sea' with its last letter removed is 'se'.
'gee'+'se'='GEESE'
(Other definitions for geese that I've seen before include "Protectors of city" , "Edible water birds" , "Birds that fly in a skein" , "Domesticated birds" , "Large water-birds with webbed feet and broad bills" .)