Coke perhaps finally spilled on mat (4)
I believe the answer is:
drug
'coke perhaps' is the definition.
(coke is a kind of drug)
'finally spilled on mat' is the wordplay.
'finally' suggests the final letters.
'on' says to put letters next to each other.
'mat' becomes 'rug' (I've seen this before).
The last letter of 'spilled' is 'd'.
'd'+'rug'='DRUG'
(Other definitions for drug that I've seen before include "Perhaps grass" , "Chemical used for medical reasons" , "Chemical medicament" , "Form of medicine" , "Medication" .)