Perhaps oxygen fix good when breathing with difficulty (7)
I believe the answer is:
gasping
'breathing with difficulty' is the definition.
'perhaps oxygen fix good' is the wordplay.
'perhaps oxygen' becomes 'gas'.
'fix' becomes 'pin' (pinning is a kind of fixing).
'good' becomes 'g' (abbreviation).
'gas'+'pin'+'g'='GASPING'
'when' is the link.
(Other definitions for gasping that I've seen before include "Showing amazement" , "Breathe noisily" .)