Discovers, when one comes upon unconscious (5,3)
I believe the answer is:
finds out
'discovers' is the definition.
('find out' can be a synonym of 'discover')
'comes upon unconscious' is the wordplay.
'comes upon' becomes 'finds' ('find' can be a synonym of 'come upon').
'unconscious' becomes 'out' (similar in meaning).
'finds'+'out'='FINDS OUT'
'when one' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Another definition for finds out that I've seen is " Gets information about, discovers".)