Club — one by river, one incomplete (4)
I believe the answer is:
iron
'club' is the definition.
(type of golf club)
'one by river one incomplete' is the wordplay.
'one' becomes 'i' (Roman numeral).
'by' says to put letters next to each other.
'river' becomes 'r' (abbreviation for river).
'incomplete' means to remove the last letter.
'one' with its last letter taken away is 'on'.
'i'+'r'+'on'='IRON'
(Other definitions for iron that I've seen before include "Heavy metallic element" , "rations in emergency?" , "Wellington was known as the .... Duke" , "Smooth - metal" , "Remove creases from" .)