Beer makes you rather fat (5)
I believe the answer is:
stout
I believe this clue is a double definition.
'beer' is the first definition.
(I've seen this before)
'rather fat' is the second definition.
(I've seen this before)
'makes you' acts as a link.
(Other definitions for stout that I've seen before include "Rather fat; determined" , "pint" , "tipple" , "Dark beer for the rather fat" , "Resolute" .)