Creature, rumour has it, needs oats primarily (5)
I believe the answer is:
rhino
'creature' is the definition.
(I have seen 'Horned creature ' mean 'rhino' so perhaps 'creature' could also mean 'rhino')
'rumour has it needs oats primarily' is the wordplay.
'primarily' suggests taking the first letters.
The initial letters of 'rumour has it needs oats' is 'RHINO'.
(Other definitions for rhino that I've seen before include "Horned jungle beast" , "Dosh (slang)" , "This beast may make a charge" , "Charging animal" , "Animal hunted for its horn" .)