River runs on high land mostly (5)
I believe the answer is:
tiber
'river' is the definition.
(River Tiber in Italy)
'runs on high land mostly' is the wordplay.
'runs' becomes 'r' (cricket abbreviation).
'on' means one lot of letters go next to another.
'high land' becomes 'tibet' (high-altitude region).
'mostly' means to remove the last letter (most but not all of the word).
'tibet' with its last letter removed is 'tibe'.
'r' put after 'tibe' is 'TIBER'.
(Other definitions for tiber that I've seen before include "Roman river" , "divides Rome" , "The flower of Rome" , "Rome's river" , "Italian banker" .)