Ponder over water source feeding river (5,2)
I believe the answer is:
dwell on
'ponder over' is the definition.
(to dwell on something is to ponder it)
'water source feeding river' is the wordplay.
'water source' becomes 'well' (I've seen this before).
'feeding' means one lot of letters goes inside another.
'river' becomes 'Don' (Don is an example).
'well' placed into 'don' is 'DWELL ON'.
(Other definitions for dwell on that I've seen before include "Linger over" , "Mull over" , "Think at length about" , "Ponder about" , "Speak at length about" .)