It’s proportionate to air, perhaps (5)
I believe the answer is:
ratio
'it's proportionate' is the definition.
I can't tell whether this definition defines the answer.
'to air perhaps' is the wordplay.
'perhaps' is an anagram indicator.
'to'+'air'='toair'
'toair' with letters rearranged gives 'RATIO'.
Can you help me to learn more?
(Other definitions for ratio that I've seen before include "Eg, 4 to 3" , "One to one - maybe" , "Comparative measure" , "One-to-one, maybe" , "Proportional measure" .)