After fruit drink, book entertains those who like to wallow (14)

'those who like to wallow' is the definition.
(animals who wallow in mud)

'after fruit drink book entertains' is the wordplay.
'after' means one lot of letters go next to another.
'fruit' becomes 'hip' (the fruit of a rose plant).
'drink book' becomes 'popot' (I am not sure about this - if you are sure you should believe this answer much more).
'entertains' becomes 'amuses' (amusing is a kind of entertaining).
'hip' put next to 'popot' is 'hippopot'.
'hippopot'+'amuses'='HIPPOPOTAMUSES'