Largest moon of Neptune (6)
I believe the answer is:
'largest moon of neptune' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for triton that I've seen before include "Greek sea-god like man with fish's tail, trident and shelltrumpet" , "Trot in after sea-god, son of Poseidon" , "Satellite of Neptune - minor sea god" , "Moon" , "conch-blowing deity" .)