A warehouse robbed of Old Master (5)
I believe the answer is:
adept
'master' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for adept that I've seen before include "Accomplished" , "Thoroughly proficient" , "Deacon" , "clever" , "Having the skill required" .)