Children's game next to American river (5)
I believe the answer is:
tagus
'river' is the definition.
(Tagus is an example)
'children's game next to american' is the wordplay.
'children's game' becomes 'tag' (I've seen this before).
'next to' is a charade indicator (letters next to each other).
'american' becomes 'us'.
'tag'+'us'='TAGUS'
(Other definitions for tagus that I've seen before include "Longest river of Iberian peninsular" , "European river" , "waterway" , "Current" , "Lisbon's river" .)