Swimming in her river (5)
I believe the answer is:
rhine
'river' is the definition.
(Rhine River is an example)
'swimming in her' is the wordplay.
'swimming' indicates an anagram (letters swim around into a new order).
'in'+'her'='inher'
'inher' is an anagram of 'RHINE'.
(Other definitions for rhine that I've seen before include "valley" , "fed by 8 [RHUR]" , "Cologne's river" , "River at Bonn, Koln" , "Great European river with Lorelei Rock" .)