Uncommon fellow may finally thin out (6)
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" .)