Unfinished tomb coated in marble compound (9)
I believe the answer is:
aggravate
'compound' is the definition.
(I've seen this before)
'unfinished tomb coated in marble' is the wordplay.
'unfinished' means to remove the last letter.
'tomb' becomes 'grave' (synonyms).
'coated in' is an insertion indicator.
'marble' becomes 'agate' (I've seen this before).
'grave' with its final letter removed is 'grav'.
'grav' going into 'agate' is 'AGGRAVATE'.
(Other definitions for aggravate that I've seen before include "Pester" , "Worsen" , "Annoy; make worse" , "Make a bad situation worse" , "compound" .)