Conventional source of water and power involving river (10)
I believe the answer is:
mainstream
'conventional' is the definition.
(I've seen this before)
'source of water and power involving river' is the wordplay.
'source of water' becomes 'main' (I've seen this before).
'and' is a charade indicator (letters next to each other).
'power' becomes 'steam' (I've seen this before).
'involving' means one lot of letters goes inside another.
'river' becomes 'r'.
'steam' placed around 'r' is 'stream'.
'main'+'stream'='MAINSTREAM'
(Other definitions for mainstream that I've seen before include "In accordance with what is normal or standard" , "conventional activities" , "People representing most common ideas" , "Conventional, normal" , "Current" .)