Fatter as a result of beer — pounds one’s hiding (8)
I believe the answer is:
portlier
'fatter' is the definition.
('portly' can be similar in meaning to 'fat')
'beer pounds one's hiding' is the wordplay.
'beer' becomes 'porter' (type of beer).
'pounds one's' becomes 'li' (I can't justify this - if you can you should give a lot more credence to this answer).
'hiding' means one lot of letters goes inside another.
'porter' placed around 'li' is 'PORTLIER'.
'as a result of' acts as a link.
Can you help me to learn more?
(Other definitions for portlier that I've seen before include "Larger" , "more stout" .)