Lord, soldier and best friend of royalty (5)
I believe the answer is:
corgi
'best friend of royalty' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'lord soldier' is the wordplay.
'lord' becomes 'cor' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'soldier' becomes 'GI' (GI can mean a US soldier).
'cor'+'gi'='CORGI'
'and' acts as a link.
Can you help me to learn more?
(Other definitions for corgi that I've seen before include "Little dog" , "Welsh breed of short-legged dog" , "Cardigan or Pembroke dog" , "HM's favourite dog" , "Pre-2018 royal dog" .)