One runs for a long time (3)
I believe the answer is:
eon
'long time' is the definition.
(I know that eon is a type of long time)
'one runs' is the wordplay.
'runs' indicates anagramming the letters.
'one' is an anagram of 'EON'.
'for a' is the link.
(Other definitions for eon that I've seen before include "In time, long division" , "Prolonged period" , "Geological period" , "An immense time" , "Vast age; eternity" .)