Possibly relaying by runner (8)
I believe the answer is:
yearling
'runner' is the definition.
(young race horse, something which runs)
'possibly relaying' is the wordplay.
'possibly' indicates an anagram.
'relaying' is an anagram of 'YEARLING'.
'by' is the link.
(Other definitions for yearling that I've seen before include "Animal between 1 and 2 in age" , "Young horse" , "Animal aged one" , "Animal 12 months old" , "Young filly, perhaps" .)