Convince it is, in short, fine within state (7)
I believe the answer is:
satisfy
'convince' is the definition.
(thesaurus)
'it is in short fine within state' is the wordplay.
'it is' becomes 'tis'.
'in' indicates putting letters inside.
'short' indicates taking the first letters.
'within' indicates putting letters inside.
'state' becomes 'say' ('say' can be a synonym of 'state'**).
The first letter of 'fine' is 'f'.
'f' inserted within 'say' is 'safy'.
'tis' placed within 'safy' is 'SATISFY'.
(Other definitions for satisfy that I've seen before include "Content" , "Meet, comply with" , "Meet; please" , "Fully meet (conditions)" , "Quench, sate" .)