Random opium use not ultimately causing rash (9)
I believe the answer is:
impetuous
'rash' is the definition.
(thesaurus)
'random opium use not ultimately' is the wordplay.
'random' indicates anagramming the letters.
'ultimately' suggests the final letters.
The final letter of 'not' is 't'.
'opium'+'use'+'t'='opiumuset'
'opiumuset' is an anagram of 'IMPETUOUS'.
'causing' is the link.
(Other definitions for impetuous that I've seen before include "Acting rashly" , "Hot-headed" , "Impulsive, hasty" , "Rash" .)