The bee in one's bonnet? (7)
I believe the answer is:
initial
'the bee' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'in one's bonnet?' is the wordplay.
'one' becomes 'i' (Roman numeral).
'bonnet?' becomes 'tial' (I can't justify this - if you can you should give a lot more credence to this answer).
'in'+'i'+'tial'='INITIAL'
Can you help me to learn more?
(Other definitions for initial that I've seen before include "Introductory" , "Primary" , "shortened version of my name" , "Letter - opening" , "Relating to the beginning" .)