Sue is so proper (7)
I believe the answer is:
solicit
'sue' is the definition.
(thesaurus)
'so proper' is the wordplay.
'proper' becomes 'licit'.
'so'+'licit'='SOLICIT'
'is' is the link.
(Other definitions for solicit that I've seen before include "Request, induce" , "Ask (someone) for something" , "Ask (favour)" , "Petition - importune" , "Request urgently and/or persistently" .)