Run after a man a long time! (6)
I believe the answer is:
manage
'run' is the definition.
(I've seen this before)
'after a man a long time' is the wordplay.
'after' means one lot of letters go next to another.
'a long time' becomes 'age' ('age' can be a synonym of 'long time').
'man'+'age' is 'MANAGE'.
(Other definitions for manage that I've seen before include "Be in charge of something" , "Take control" , "Be in overall control of" , "Control; cope" , "Run; cope" .)