Modern painting restorers are concerned with this (5-2-3-3)
I believe the answer is:
state-of-the-art
'modern' is the definition.
(I've seen this before)
'painting restorers are concerned with this' is the wordplay.
I cannot really understand how this works, but
'painting' could be 'art' (painting is an art) and 'art' is present in the answer.
'are' could be 'a' (short for 'are', historical unit of measurement) and 'a' is located in the answer.
'concerned with' could be 'of' ('of' can mean 'concerned with' or 'relating to') and 'of' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for state-of-the-art that I've seen before include "Produced by the latest technology" , "Incorporating the latest technologies" , "Using latest technology" , "Modern" , "Up-to-the-minute" .)