55 in river search (5)
I believe the answer is:
delve
'search' is the definition.
(to delve into something is to search through it)
'55 in river' is the wordplay.
'55' becomes 'LV' (Roman numerals).
'in' is an insertion indicator.
'river' becomes 'dee' (River Dee).
'lv' going within 'dee' is 'DELVE'.
(Other definitions for delve that I've seen before include "Look closely (into)" , "Hollow" , "Make painstaking enquiries (into something)" , "probe deeply" , "Turn up - dig" .)