Frolicking in her river (5)
I believe the answer is:
rhine
'river' is the definition.
(Rhine is an example)
'frolicking in her' is the wordplay.
'frolicking' is an anagram indicator.
'in'+'her'='inher'
'inher' anagrammed gives 'RHINE'.
(Other definitions for rhine that I've seen before include "River at Bonn, Koln" , "Major European river" , "Great river in Germany with Lorelei Rock" , "Cologne's river" , "Bonn's river" .)