Number five brought into the world, inhaling oxygen (5)
I believe the answer is:
boron
'number five' is the definition.
(element with the atomic number 5)
'brought into the world inhaling oxygen' is the wordplay.
'brought into the world' becomes 'born' (I've seen this before).
'inhaling' indicates putting letters inside.
'oxygen' becomes 'o' (atomic symbol for oxygen).
'born' enclosing 'o' is 'BORON'.
(Other definitions for boron that I've seen before include "Atomic no 5" , "element needed for steel" , "component of steel" , "Non-metallic element, symbol B" , "Chemical element, symbol B" .)