Briefly arrive at a state of unconsciousness (4)
I believe the answer is:
coma
'state of unconsciousness' is the definition.
(I've seen this in another clue)
'briefly arrive at a' is the wordplay.
'briefly' means to remove the last letter.
'arrive' becomes 'come' (coming is a kind of arriving).
'at' says to put letters next to each other.
'come' with its last letter removed is 'com'.
'com'+'a'='COMA'
(Other definitions for coma that I've seen before include "State of unconsciousness caused by illness" , "Long period of unconsciousness" , "long sleep" , "Unconscious state" , "Unconsciousness from which person cannot be aroused" .)