To canvass is so legal! (7)
I believe the answer is:
solicit
'to canvass' is the definition.
(thesaurus)
'so legal' is the wordplay.
'legal' becomes 'licit' (similar in meaning).
'so'+'licit'='SOLICIT'
'is' is the link.
(Other definitions for solicit that I've seen before include "Beg - hustle" , "Tout" , "Try to obtain (something) from someone" , "Drum up business" , "Accost - request" .)