Canine, often royal, growls intimidatingly, initially (5)
I believe the answer is:
corgi
'canine' is the definition.
(I know that corgi is a type of dog)
'often royal growls intimidatingly initially' is the wordplay.
'often royal' becomes 'cor' (I can't justify this - if you can you should give a lot more credence to this answer).
'initially' indicates taking the first letters.
The first letters of 'growls intimidatingly' is 'gi'.
'cor'+'gi'='CORGI'
Can you help me to learn more?
(Other definitions for corgi that I've seen before include "The Queen's favourite?" , "Toy car manufacturer" , "Cardigan or Pembroke dog" , "man's best friend" , "Barker?" .)