Sovereign's beginning to appear with piece of regalia for someone in Lusatia (4)
I believe the answer is:
sorb
'someone in lusatia' is the definition.
I can't judge whether this defines the answer.
'sovereign's beginning to appear with piece of regalia for' is the wordplay.
'beginning to' suggests taking the first letters.
'appear with' indicates anagramming the letters.
'piece' becomes 'r' (abbreviation for Rook).
'of' becomes 'o''.
'regalia for' becomes 'b' (I am not sure about this - if you are sure you should believe this answer much more).
The first letter of 'sovereigns' is 's'.
'r'+'o'='ro'
's' anagrammed with 'ro' gives 'sor'.
'sor'+'b'='SORB'
Can you help me to learn more?
(Another definition for sorb that I've seen is " Service tree, its fruit".)