Appearing sleepless, British LSD advocate Observer finally featured (6-4)
I believe the answer is:
bleary-eyed
'appearing' is the definition.
Both the answer and definition are adjectives. Maybe you can see an association between them that I can't see?
'sleepless british lsd advocate observer finally featured' is the wordplay.
'sleepless british lsd advocate' becomes 'bleary' (I can't justify this - if you can you should give a lot more credence to this answer).
'observer' becomes 'eye' (organ which sees or observes).
'finally' indicates one should take the final letters.
The final letter of 'featured' is 'd'.
'bleary'+'eye'+'d'='BLEARY-EYED'
Can you help me to learn more?
(Other definitions for bleary-eyed that I've seen before include "Unperceptive" , "Obviously tired" , "Looking sleepy" , "Looking very tired" , "Looking half-awake" .)