Chief decorator's last undercoat (6)
I believe the answer is:
primer
'undercoat' is the definition.
(preliminary coat of paint)
'chief decorator's last' is the wordplay.
'chief' becomes 'prime' (I've seen this in another clue).
'decorator's last' becomes 'r' (I am not sure about this - if you are sure you should believe this answer much more).
'prime'+'r'='PRIMER'
Can you help me to learn more?
(Other definitions for primer that I've seen before include "Basic teaching book" , "Preparatory coat of paint" , "He charges" , "introductory book" , "Introductory textbook or first coat of paint" .)