Training her in river (5)
I believe the answer is:
'river' is the definition.
(Rhine is an example)
'training her in' is the wordplay.
'training' indicates anagramming the letters (to train can mean to grow something into a shape).
'herin' anagrammed gives 'RHINE'.
(Other definitions for rhine that I've seen before include "Great German river with Lorelei Rock" , "Major European river" , "Cologne's river" , "Great European river with Lorelei Rock" , "Great river that flows through Germany" .)