Patient’s originally malign agent checked by hospital oxygen? (8)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

A clue with the main definition (as part of an &lit) in the middle: a charade of P (‘ P atient’s originally’) plus ATHOGEN, an envelope (‘checked by’) of H (‘hospital’) plus O (‘oxygen’) in ATGEN, an anagram (‘malign’) of ‘agent’.