Beast Romeo initially humble in love (5)
I believe the answer is:
rhino
'beast' is the definition.
(I've seen this before)
'romeo initially humble in love' is the wordplay.
'romeo' becomes 'r' (phonetic alphabet: alpha, bravo, charlie etc.).
'initially' suggests taking the first letters.
'love' becomes 'o' (love means zero in tennis).
The first letter of 'humble' is 'h'.
'r'+'h'+'in'+'o'='RHINO'
(Other definitions for rhino that I've seen before include "Cash" , "Ready" , "Horned animal; money (slang)" , "Thick-skinned type" , "Endangered mammal" .)