Explore large volume in river (5)
I believe the answer is:
delve
'explore' is the definition.
(I've seen this in another clue)
'large volume in river' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'volume' becomes 'v' (abbreviation for volume in book titles).
'in' is an insertion indicator.
'river' becomes 'Dee' (River Dee).
'l'+'v'='lv'
'lv' placed into 'dee' is 'DELVE'.
(Other definitions for delve that I've seen before include "Dig - research deeply" , "probe deeply" , "Dig - rummage" , "Turn up - dig" , "Dig up - search" .)