One on a quest (6)
I believe the answer is:
seeker
'one on a quest' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for seeker that I've seen before include "One attempting to find" , "Inquirer; quester" , "One is looking" , "One who pursues and searches for" , "Missile target locator" .)