Cover up over damage by unknown drug dealer (8)
I believe the answer is:
pharmacy
'drug dealer' is the definition.
(I've seen this before)
'cover up over damage' is the wordplay.
'cover up' becomes 'pacy' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'over' means one lot of letters goes inside another.
'damage' becomes 'harm' (synonyms).
'pacy' going around 'harm' is 'PHARMACY'.
'by unknown' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for pharmacy that I've seen before include "Where drugs are dispensed" , "Dispensary" , "get medicine here?" , "It supplies drugs" , "A chemist's shop where prescriptions are dispensed" .)