One wallows in river (5)
I believe the answer is:
hippo
'one wallows' is the definition.
(hippopotamuses wallow in water)
'in river' is the wordplay.
'in' becomes 'hip' (both can mean fashionable).
'river' becomes 'Po' (Italian river).
'hip'+'po'='HIPPO'
(Other definitions for hippo that I've seen before include "One thick-skinned" , "one that's large and fleshy" , "Large semi-aquatic pachyderm (abbr)" , "African river mammal" , "large African creature" .)