Further damage a length of metamorphic rock (9)
I believe the answer is:
marmoreal
'of metamorphic rock' is the definition.
(marmoreal means relating to marble)
'further damage a length' is the wordplay.
I cannot quite see how this works, but
'further' could be 'more' (synonyms) and 'more' is present in the answer.
'damage' could be 'mar' (to mar is to spoil or damage) and 'mar' is located in the answer.
'a' is present in the answer.
'length' could be 'l' (abbreviation in geometry) and 'l' is present in the answer.
No letters remain.
This explanation may well be incorrect...
(Other definitions for marmoreal that I've seen before include "stony cold" , "made of stone" , "of fine stone" .)