Think constantly about source of water in river (5,2)
I believe the answer is:
dwell on
'think constantly about' is the definition.
I can't judge whether this defines the answer.
'source of water in river' is the wordplay.
'source of water' becomes 'well' (water may come from a well).
'in' indicates putting letters inside.
'river' becomes 'Don' (Yorkshire river).
'well' put within 'don' is 'DWELL ON'.
Can you help me to learn more?
(Other definitions for dwell on that I've seen before include "Speak at length about" , "Linger over" , "Mull over" , "Ponder about" , "Think at length about" .)