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 anagramming the letters.
'relaying' is an anagram of 'YEARLING'.
'by' acts as a 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" .)