Filly, one that's running (8)
I believe the answer is:
galloper
'running' is the definition.
Both the answer and definition are singular nouns.
Maybe you can see an association between them that I can't see?
'filly one' is the wordplay.
'filly' becomes 'gal' (I've seen this in another clue).
'one' becomes 'loper' (I am not sure about this - if you are sure you should believe this answer much more).
'gal'+'loper'='GALLOPER'
'that's' is the link.
Can you help me to learn more?
(Other definitions for galloper that I've seen before include "Sprinter" , "fast mover" .)