Singer failing to inhale oxygen (5)
I believe the answer is:
voice
'singer' is the definition.
(I know that voice is a type of singer)
'failing to inhale oxygen' is the wordplay.
'failing' becomes 'vice' (vice can mean a moral failing).
'to inhale' indicates putting letters inside.
'oxygen' becomes 'O' (atomic symbol for oxygen).
'vice' placed around 'o' is 'VOICE'.
(Other definitions for voice that I've seen before include "tenor, say" , "(Human) utterance" , "Sound of speaker" , "Ability to speak, sing" , "Speak of - mouthpiece" .)