Secure on river bank (4)
I believe the answer is:
tier
'bank' is the definition.
(both can mean a row of similar things)
'secure on river' is the wordplay.
'secure' becomes 'tie' (tying is a kind of securing).
'on' says to put letters next to each other.
'river' becomes 'r' (abbreviation for river).
'tie'+'r'='TIER'
(Other definitions for tier that I've seen before include "this person secures" , "he hasn't won?" , "Row, level" , "Rite for a rank" , "Wedding cake feature" .)