In favour of bottom of river being delved into (6)
I believe the answer is:
probed
'being delved into' is the definition.
(I've seen this in another clue)
'in favour of bottom of river' is the wordplay.
'in favour of' becomes 'pro' (being pro a particular cause).
'bottom of river' becomes 'bed' (I've seen this before).
'pro'+'bed'='PROBED'
(Other definitions for probed that I've seen before include "Investigating closely" , "Questioned or examined thoroughly and closely" , "Physically examined" , "Investigated closely as dentist does a tooth" , "Searched" .)