Mixer is briefly, perhaps, wide open (9)
I believe the answer is:
extrovert
'mixer' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'briefly perhaps wide open' is the wordplay.
'briefly' means to remove the last letter.
'perhaps wide' becomes 'extra' (I've seen this before).
'open' becomes 'overt' (synonyms).
'extra' with its final letter removed is 'extr'.
'extr'+'overt'='EXTROVERT'
'is' is the link.
Can you help me to learn more?
(Other definitions for extrovert that I've seen before include "show-off" , "he's turned out" , "Person who is more interested in the world than the inner self" , "Outgoing rather than inward-looking" , "Outgoing (personality)" .)