Rest at inn unexpectedly brief (9)
I believe the answer is:
transient
'brief' is the definition.
(thesaurus)
'rest at inn unexpectedly' is the wordplay.
'unexpectedly' indicates an anagram.
'rest'+'at'+'inn'='restatinn'
'restatinn' with letters rearranged gives 'TRANSIENT'.
(Other definitions for transient that I've seen before include "Lasting a short time" , "Here today, gone tomorrow" , "fugitive" , "Passing, short-lived" , "Ephemeral" .)