Bee's protector isn't terribly grand (5)
I believe the answer is:
sting
'bee's protector' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'isn't terribly grand' is the wordplay.
'terribly' indicates an anagram.
'grand' becomes 'g' (G is short for grand as in a thousand).
'isnt' is an anagram of 'stin'.
'stin'+'g'='STING'
Can you help me to learn more?
(Other definitions for sting that I've seen before include "Undercover operation; wound" , "Painful wound - confidence trick" , "(Bee) wound" , "Wound from wasp" , "aka Gordon Sumner" .)