It may be stolen on a diamond (4)
I believe the answer is:
base
'it may be stolen on a diamond' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for base that I've seen before include "Ignoble; low part" , "Interest charged by the Bank of England to commercial banks" , "Shameful - headquarters" , "Found - station - contemptible" , "unprincipled" .)