Poseidon (7)
I believe the answer is:
neptune
'poseidon' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for neptune that I've seen before include "Roman god of the sea, with trident" , "Holst composed it" , "heavenly body" , "part of Planets Suite" , "Eighth rock from the Sun" .)