Sea god in Greek mythology; a son of Poseidon and Amphitrite (6)
I believe the answer is:
triton
(Other definitions for triton that I've seen before include "conch-blowing deity" , "Greek minor sea-god with trident, fish-tail and shell trumpet" , "Moon" , "Horn-blower" , "Greek sea-god like man with fish's tail, trident and shelltrumpet" .)