Largest moon of Neptune (6)
I believe the answer is:
triton
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">largest moon of neptune</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)