Run after a man a long time! (6)
I believe the answer is:
manage
'run' is the definition.
(run can mean to manage or be in charge of)
'after a man a long time' is the wordplay.
'after' means one lot of letters go next to another.
'a long time' becomes 'age' (an age is a period of 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" .)