The eighth of eight (7)
I believe the answer is:
neptune
'the eighth of eight' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for neptune that I've seen before include "Roman god of the sea, with trident" , "Eighth planet from the Sun" , "in Holst's suite" , "Tune pen into the Roman god" , "Sea god - outer planet" .)