Jarring sound principally associated with Neptune's spear (8)
I believe the answer is:
strident
'jarring' is the definition.
(thesaurus)
'sound principally associated with neptune's spear' is the wordplay.
'principally' says to take the initial letters.
'associated with' says to put letters next to each other.
'neptune's spear' becomes 'trident' (I've seen this in another clue).
The first letter of 'sound' is 's'.
's'+'trident'='STRIDENT'
(Other definitions for strident that I've seen before include "Loud and vociferous" , "Raucous" , "Clamorous" , "Tumultuous" , "Unpleasantly rough" .)