Highly intoxicating (5)
I believe the answer is:
heady
'intoxicating' is the definition.
(synonyms)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for heady that I've seen before include "Potent, intoxicating" , "Impetuous" , "Inebriating" , "Extremely exciting" .)