Run hard alongside one river (5)
I believe the answer is:
rhone
'river' is the definition.
(I know that Rhone is a river)
'run hard alongside one' is the wordplay.
'run' becomes 'r' (cricket abbreviation).
'hard' becomes 'h' (abbreviation used in pencil classifications).
'alongside' says to put letters next to each other.
'r'+'h'+'one'='RHONE'
(Other definitions for rhone that I've seen before include "River rising in Switzerland and flowing into the Mediterranean" , "French flower" , "River flowing through Lake Geneva" , "French river from Alps to Mediterranean" , "River running from the Swiss Alps to the Mediterranean" .)