All the players or just one runner (6)
I believe the answer is:
castor
'one runner' is the definition.
The answer and definition can be both animals as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'all the players or' is the wordplay.
'all the players' becomes 'cast' (group of actors).
'cast'+'or'='CASTOR'
'just' acts as a link.
Can you help me to learn more?
(Other definitions for castor that I've seen before include "One star" , "Pollux's twin" , "Twin to Pollux" , "Sprinkler" , "Furniture wheel; sort of oil" .)