Crossword Genius

Uncommon fellow may finally thin out (6)

Ross

I believe the answer is:

rarefy

'thin out' is the definition.
(I've seen this before)

'uncommon fellow may finally' is the wordplay.
'uncommon' becomes 'rare' (similar in meaning).
'fellow' becomes 'f' (abbreviation - of a society etc.).
'finally' suggests the final letters.
The last letter of 'may' is 'y'.
'rare'+'f'+'y'='RAREFY'

(Other definitions for rarefy that I've seen before include "Fry are (anag.)" , "Refine" , "Make less solid; a ferry (anag.)" , "Become less dense" , "Make thinner" .)

I've seen this clue in the Sydney Morning Herald.
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