Relationship to air travellers (5)
I believe the answer is:
ratio
'relationship' is the definition.
(relationship between two numbers)
'to air travellers' is the wordplay.
'to air' can be anagrammed to 'RATIO'.
However, I'm unsure how this is indicated.
Can you help me to learn more?
(Other definitions for ratio that I've seen before include "Mathematical relation of one thing to another" , "Relationship found in a riot" , "five to four, perhaps" , "Eg, 4 to 3" , "Comparative size" .)