One ran 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 ran' is the wordplay.
'ran' is an anagram indicator (letters have run around).
'one' with letters rearranged gives 'EON'.
'for a' is the link.
(Other definitions for eon that I've seen before include "Very long period (US and technical spelling)" , "thousand million years" , "Immense period" , "Vast age (alt. sp.)" , "Very long age" .)