Experienced people keeping peace on river (9)
I believe the answer is:
underwent
'experienced people' is the definition.
'underwent' can be an answer for 'experienced' (I've seen this before). I'm not sure about the 'people' bit.
'peace on river' is the wordplay.
'peace on' becomes 'un' (I can't explain this - if you can you should give a lot more credence to this answer).
'river' becomes 'derwent' (I've seen this before).
'un'+'derwent'='UNDERWENT'
'keeping' acts as a link.
Can you help me to learn more?
(Other definitions for underwent that I've seen before include "Was subjected to" , "Experienced; bore" .)