Electrical discharge in large vessel reported (3)
I believe the answer is:
arc
'electrical discharge' is the definition.
(I've seen this before)
'large vessel reported' is the wordplay.
'large vessel' becomes 'ark' (I've seen this before).
'reported' indicates a 'sounds like' (homophone) clue.
'ark' sounds like 'ARC'.
'in' is the link.
(Other definitions for arc that I've seen before include "Semi-circle" , "Part of a circle's circumference" , "Half-moon" , "Something curved in shape" , "Light" .)