Street vendor ultimately protects miserable proprietor (9)
I believe the answer is:
landowner
'proprietor' is the definition.
(thesaurus)
'street vendor ultimately protects miserable' is the wordplay.
'street' becomes 'lane'.
'ultimately' says to take the final letters.
'protects' is an insertion indicator (outer letters protect inside letters).
'miserable' becomes 'down'.
The last letter of 'vendor' is 'r'.
'lane'+'r'='laner'
'laner' enclosing 'down' is 'LANDOWNER'.
(Other definitions for landowner that I've seen before include "man of property" , "One with much property" , "Man of means" , "Possessor of real estate" , "Laird" .)