Weller, perhaps, at first upset regularly erratic retainer (7)
I believe the answer is:
samurai
'retainer' is the definition.
The definition and answer can be both people as well as being singular nouns.
Perhaps you can see an association between them that I don't see?
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for samurai that I've seen before include "Japanese warrior code" , "Japanese militarist" , "Japanese military retainer" , "Sudoku variant" , "Member of military class" .)