Wallower in trendy popular music, returning to entertain endlessly (12)
I believe the answer is:
hippopotamus
'wallower' is the definition.
(animal that wallows in mud)
'trendy popular music returning to entertain endlessly' is the wordplay.
'trendy' becomes 'hip' (I've seen this before).
'popular music' becomes 'pop'.
'returning' says the letters should be written in reverse.
'entertain' becomes 'amuse' (amusing is a kind of entertaining).
'endlessly' means to remove the last letter.
'to' written backwards gives 'ot'.
'amuse' with its last letter taken away is 'amus'.
'hip'+'pop'+'ot'+'amus'='HIPPOPOTAMUS'
'in' acts as a link.
(Other definitions for hippopotamus that I've seen before include "African animal" , "River horse" , "Oppo hams it up (anag)" , "African river beast" , "African ungulate" .)