To canvass is thus perfectly legal (7)
I believe the answer is:
solicit
'to canvass' is the definition.
(thesaurus)
'thus perfectly legal' is the wordplay.
'thus' becomes 'so' ('so' can be a synonym of 'thus').
'perfectly legal' becomes 'licit'.
'so'+'licit'='SOLICIT'
'is' acts as a link.
(Other definitions for solicit that I've seen before include "Invite, beg for" , "Accost - request" , "Ask earnestly about colitis" , "Request urgently and/or persistently" , "Request, induce" .)