Fellow has to go on runs (4)
I believe the answer is:
peer
'fellow' is the definition.
(thesaurus)
'to go on runs' is the wordplay.
'to go' becomes 'pee' (both can mean to urinate**).
'on' is a charade indicator (letters next to each other).
'runs' becomes 'r' (cricket abbreviation).
'pee'+'r'='PEER'
'has' acts as a link.
(Other definitions for peer that I've seen before include "Look closely at equal" , "Struggle to see; equal" , "Equal - match" , "Aristocrat" , "Equal; nobleman" .)