A growing source of hot air (4)
I believe the answer is:
'a growing' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'source of hot air' is the wordplay.
'source of' suggests taking the first letters.
The first letter of 'hot' is 'h'.
Can you help me to learn more?
(Other definitions for hair that I've seen before include "Filament growing from the skin" , "Coiffure" , "Fibre; small extent" , "Musical - locks" , "Nineteen sixties musical; filament" .)