Temporary pavilion so unstable, sheltering Rex (11)
I believe the answer is:
provisional
'temporary' is the definition.
(thesaurus)
'pavilion so unstable sheltering rex' is the wordplay.
'unstable' indicates anagramming the letters.
'sheltering' is an insertion indicator.
'rex' becomes 'r' (common abbreviation e.g. GR=George Rex, King Georg).
'pavilion'+'so'='pavilionso'
'pavilionso' with letters rearranged gives 'povisional'.
'povisional' placed around 'r' is 'PROVISIONAL'.
(Other definitions for provisional that I've seen before include "Temporary - viral poison (anag)" , "Temporary - conditional" , "Temporary, interim" , "Pending subsequent decision" .)