Oxygen expired after Babel threw fit (4-6)
I believe the answer is:
able-bodied
'fit' is the definition.
(I've seen this before)
'oxygen expired after babel threw' is the wordplay.
'oxygen' becomes 'O' (atomic symbol for oxygen).
'expired' becomes 'died' ('die' can be a synonym of 'expire').
'after' means one lot of letters go next to another.
'threw' indicates anagramming the letters (I've seen 'throw' mean this).
'babel' anagrammed gives 'ableb'.
'o'+'died'='odied'
'odied' after 'ableb' is 'ABLE-BODIED'.
(Other definitions for able-bodied that I've seen before include "Not physically unfit" , "Physically strong and healthy" .)