Providing oxygen, say, also supply nitrogen at first (2,4,2)
I believe the answer is:
as long as
'providing' is the definition.
I know nothing about this answer so I can't tell whether this works.
'oxygen say also supply nitrogen at first' is the wordplay.
'oxygen say' becomes 'gas' (oxygen is a kind of gas).
'supply' indicates anagramming the letters (in a supple way).
'nitrogen' becomes 'n' (N is the chemical symbol for nitrogen).
'at first' means one lot of letters go next to another.
'also' with letters rearranged gives 'aslo'.
'aslo'+'n'='aslon'
'gas' after 'aslon' is 'AS LONG AS'.
Can you help me to learn more?
(Other definitions for as long as that I've seen before include "On condition that - (" ___ your arm")" , "Provided" , "for the time" .)