Rocky outcrop beside river (3)
I believe the answer is:
tor
'rocky outcrop' is the definition.
(I've seen this before)
'beside river' is the wordplay.
'beside' becomes 'to' (I can't explain this - if you can you should give a lot more credence to this answer).
'river' becomes 'r'.
'to'+'r'='TOR'
Can you help me to learn more?
(Other definitions for tor that I've seen before include "High, rocky, bare hill" , "fell" , "rise" , "eminence" , "Pillar of rock" .)