Complaint of old lady needing to inhale oxygen (5)
I believe the answer is:
groan
'complaint' is the definition.
(to groan is to complain about something)
'old lady needing to inhale oxygen' is the wordplay.
I cannot really see how this works, but
'old' could be 'o' (common abbreviation eg in OE for Old English) and 'o' is located in the answer.
'lady' could be 'gran' (I have seen 'old lady' mean 'gran' so perhaps 'lady' could also mean 'gran') and 'gran' is present in the remaining letters.
This accounts for all the letters.
This explanation may well be incorrect...
'of' is the link.
Can you help me to learn more?
(Other definitions for groan that I've seen before include "Express disappointment, pain" , "Titus -- (Mervyn Peake)" , "Sound of pain or disapproval" , "Expression of pain or disapproval" , "Sound of pain or despair" .)