A river to label American (5)
I believe the answer is:
tagus
'a river' is the definition.
(Tagus is an example)
'label american' is the wordplay.
'label' becomes 'tag' ('tag' can be a synonym of 'label'**).
'american' becomes 'us'.
'tag'+'us'='TAGUS'
'to' acts as a link.
(Other definitions for tagus that I've seen before include "European river" , "river flowing to Atlantic" , "Longest river of Iberian peninsula" , "waterway" , "river abroad" .)