It's not air, perhaps, that one may have to live on (6)
I believe the answer is:
ration
'one may have to live on' is the definition.
I can't judge whether this definition defines the answer.
'it's not air perhaps' is the wordplay.
'perhaps' indicates anagramming the letters.
'not'+'air'='notair'
'notair' is an anagram of 'RATION'.
'that' is the link.
Can you help me to learn more?
(Other definitions for ration that I've seen before include "Restricted share" , "Restrict the consumption of something" , "Share or allotment, of food say" , "Fixed restricted allowance" , "A measured portion in times of scarcity" .)