Tom's mare could be a beast (8)
I believe the answer is:
marmoset
'a beast' is the definition.
The answer and definition can be both animals as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'tom's mare could be' is the wordplay.
'could be' indicates an anagram.
'toms'+'mare'='tomsmare'
'tomsmare' anagrammed gives 'MARMOSET'.
Can you help me to learn more?
(Other definitions for marmoset that I've seen before include "Somme rat? (anag)" , "Small monkey - Rome's mat (anag)" , "Take me to arms about bushy-tailed monkey of tropical America" , "Primate" , "Small tropical American monkey" .)