Badly hurt, he’d be on oxygen, perhaps (6)
I believe the answer is:
gashed
'badly hurt' is the definition.
Both the answer and definition are adjectives. Perhaps there's an association between them I don't understand?
'he'd be on oxygen perhaps' is the wordplay.
'be on' says to put letters next to each other.
'oxygen perhaps' becomes 'gas' (oxygen is a kind of gas).
'hed' after 'gas' is 'GASHED'.
Can you help me to learn more?
(Other definitions for gashed that I've seen before include "Cut open" , "Deeply cut" , "Wounded" .)