Drop in air flow yields an unexpected profit (8)
I believe the answer is:
windfall
'profit' is the definition.
(windfall profit is a kind of profit)
'drop in air flow yields an unexpected' is the wordplay.
I cannot quite understand how this works, but
'drop' could be 'fall' (synonyms) and 'fall' is found in the answer.
'in' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for windfall that I've seen before include "unexpected blessing" , "Bruised apple" , "bonanza" , "Piece of luck (but it might be bruised)" , "Sudden and unexpected piece of good fortune" .)