Chief renderer’s first undercoat (6)
I believe the answer is:
primer
'undercoat' is the definition.
(preliminary coat of paint)
'chief renderer's first' is the wordplay.
'chief' becomes 'prime' (I've seen this before).
'first' suggests taking the first letters.
The initial letter of 'renderer' is 'r'.
'prime'+'r'='PRIMER'
(Other definitions for primer that I've seen before include "Introductory textbook or first coat of paint" , "Sealant coat" , "Preparatory coat of paint" , "Basic book; undercoat" , "small book of prayer" .)