Do a marathon in the fullness of time (4,3)
I believe the answer is:
long run
'do a' is the definition.
I can't tell whether this defines the answer.
'marathon in the fullness of time' is the wordplay.
'marathon' becomes 'long' (marathon can mean long or lengthy).
'in the fullness of time' becomes 'run' (I am not sure about this - if you are sure you should believe this answer much more).
'long'+'run'='LONG-RUN'
Can you help me to learn more?
(Other definitions for long run that I've seen before include "Marathon?" , "Extended period" , "Period of time sufficient for things to work themselves out" .)