Open and endlessly benign, perhaps (5)
I believe the answer is:
begin
'open' is the definition.
(thesaurus)
'endlessly benign perhaps' is the wordplay.
'endlessly' means to remove the last letter.
'perhaps' indicates anagramming the letters.
'benign' with its last letter taken off is 'benig'.
'benig' is an anagram of 'BEGIN'.
'and' is the link.
(Other definitions for begin that I've seen before include "Make a start" , "Israeli prime minister who shared the Nobel Peace Prize with President Sadat" , "Found" , "Initiate" , "Get cracking" .)