Secure next to river bank (4)
I believe the answer is:
tier
'bank' is the definition.
(thesaurus)
'secure next to river' is the wordplay.
'secure' becomes 'tie' (tying is a kind of securing**).
'next to' is a charade indicator (letters next to each other).
'river' becomes 'r'.
'tie'+'r'='TIER'
(Other definitions for tier that I've seen before include "Row (of seats)" , "Level of a structure" , "this person secures" , "Level within hierarchy" , "Rank, level" .)