Rat is feature on old river (7)
I believe the answer is:
traitor
'rat' is the definition.
(thesaurus)
'feature on old river' is the wordplay.
'feature' becomes 'trait' (I've seen this before**).
'on' means one lot of letters go next to another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'river' becomes 'r'.
'trait'+'o'+'r'='TRAITOR'
'is' is the link.
(Other definitions for traitor that I've seen before include "Not a good subject" , "Betrayer, deceiver" , "Betrayer of a cause" , "Betrayer of a country" , "Double-crosser" .)