Investigate entrances to locks via river banks (5)
I believe the answer is:
delve
'investigate' is the definition.
(I've seen this in another clue)
'entrances to locks via river banks' is the wordplay.
'entrances to' indicates taking the first letters (I've seen 'entrance to' mean this).
'river' becomes 'dee' (River Dee).
'banks' means one lot of letters goes inside another (bank can mean to surround or border).
The first letters of 'locks via' is 'lv'.
'lv' going into 'dee' is 'DELVE'.
(Other definitions for delve that I've seen before include "Burrow" , "Search deeply for information" , "Make painstaking enquiries (into something)" , "Reach down and search" , "Look closely (into)" .)