Minute oxygen consumer -- a cat? (6)
I believe the answer is:
mouser
'a cat?' is the definition.
(cat that kills mice)
'minute oxygen consumer' is the wordplay.
'minute' becomes 'm' (abbreviation).
'oxygen' becomes 'O' (atomic symbol for oxygen).
'consumer' becomes 'user' (user is a kind of consumer).
'm'+'o'+'user'='MOUSER'
(Other definitions for mouser that I've seen before include "Preying cat" , "Cat proficient at catching small rodents" , "One getting rid of vermin" , "Rodent catcher" , "Cat with certain skills" .)