Couple on river bank (4)
I believe the answer is:
tier
'bank' is the definition.
(both can mean a row of similar things)
'couple on river' is the wordplay.
'couple' becomes 'tie' (I've seen this in another clue).
'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 "Rite (anag.)" , "Level within hierarchy" , "Rank, level" , "Row - one forming a knot" , "Rite for a rank" .)