Fat — dark beer (5)
I believe the answer is:
stout
I believe this is a double definition.
'fat' is the first definition.
(similar in meaning)
'dark beer' is the second definition.
(I've seen this in another clue)
(Other definitions for stout that I've seen before include "Strong; drink" , "Full-bodied" , "Heavily built; beer" , "Inured to fatigue and hardship" , "tipple" .)