A growing source of hot air (4)

I believe the answer is:

hair

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'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'.
'h'+'air'='HAIR'

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" .)

I've seen this clue in the Evening Standard.

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