Skinny guy’s foremost obsession (5)
I believe the answer is:
thing
'obsession' is the definition.
(I've seen this in other clues)
'skinny guy's foremost' is the wordplay.
'skinny' becomes 'thin' (similar in meaning).
'foremost' indicates taking the first letters.
The initial letter of 'guys' is 'g'.
'thin'+'g'='THING'
(Other definitions for thing that I've seen before include "Night (anag)" , "Obsession" , "A creature" , "cup of tea" , "Object; gadget" .)