Run on to hill (3)
I believe the answer is:
tor
'hill' is the definition.
(I know that tor is a type of hill)
'run on to' is the wordplay.
'run' becomes 'r' (cricket abbreviation).
'on' says to put letters next to each other.
'r' put after 'to' is 'TOR'.
(Other definitions for tor that I've seen before include "fell" , "Dartmoor peak" , "feature of Glastonbury" , "Rot on rocky peak" , "High, rocky, bare hill" .)