Possibly being open (5)
I believe the answer is:
begin
'open' is the definition.
(thesaurus)
'possibly being' is the wordplay.
'possibly' is an anagram indicator.
'being' with letters rearranged gives 'BEGIN'.
(Other definitions for begin that I've seen before include "Found" , "Partly open" , "Israeli prime minister and Nobel Peace Prize winner, d. 1992" , "Monachem --" , "Launch" .)