Oceanic fluid that's a drug (7)
I believe the answer is:
cocaine
'that's a drug' is the definition.
(cocaine is a kind of drug)
'oceanic fluid' is the wordplay.
'fluid' indicates an anagram.
'oceanic' anagrammed gives 'COCAINE'.
(Other definitions for cocaine that I've seen before include "Addictive narcotic" , "Number" , "A narcotic, anaesthetic" , "Oceanic (anag) - drug" , "Drug from ace coin" .)