Particular eastern king's introduction to a subject (6)
I believe the answer is:
primer
'introduction to a subject' is the definition.
I can't judge whether this definition defines the answer.
'particular eastern king's' is the wordplay.
'particular' becomes 'prim' (prim can mean precise or particular).
'eastern' becomes 'e'.
'king' becomes 'R' (abbreviation of Latin rex).
'prim'+'e'+'r'='PRIMER'
Can you help me to learn more?
(Other definitions for primer that I've seen before include "Basic teaching book" , "First coat - book" , "Preparatory coat of paint" , "Educational book" , "Solitary" .)