Fishy starting price for a rodent race runner! (5)
I believe the answer is:
sprat
'race runner' is the definition.
The definition and answer can be both animals as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'fishy starting price for a rodent' is the wordplay.
'fishy starting price for' becomes 'sp' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a rodent' becomes 'rat' (rat is a kind of rodent).
'sp'+'rat'='SPRAT'
Can you help me to learn more?
(Other definitions for sprat that I've seen before include "Small fish (used to catch mackerel?)" , "Herring's smaller relative" , "Couple sharing meat" , "Small fish, sardine, say" , "Traps (anag.)" .)