In part, perhaps, one is a beast (5)
I believe the answer is:
tapir
'beast' is the definition.
(tapir is a type of animal)
'in part perhaps one' is the wordplay.
'in' indicates putting letters inside.
'perhaps' indicates anagramming the letters.
'one' becomes 'i' (Roman numeral).
'part' with letters rearranged gives 'tapr'.
'tapr' enclosing 'i' is 'TAPIR'.
'is a' is the link.
(Other definitions for tapir that I've seen before include "I part with the pig-like animal" , "Animal with elongated snout" , "Long-nosed, hoofed mammal" , "A trip (anag) - animal" , "nocturnal beast" .)