One who permits suckers to get rinsed, essentially, by old rector (8)
I believe the answer is:
licensor
'one who permits' is the definition.
(someone who permits or licenses things)
'suckers to get rinsed essentially by old rector' is the wordplay.
'suckers' becomes 'lice' (parasites that suck blood).
'to get' is a charade indicator (letters next to each other).
'essentially' means to look at the middle letters.
'by old' becomes 'o' (common abbreviation eg in OE for Old English. I am not sure about the 'by' bit.).
'rector' becomes 'r'.
The central letters of 'rinsed' are 'ns'.
'lice'+'ns'+'o'+'r'='LICENSOR'
(Other definitions for licensor that I've seen before include "Permissive type" , "who grants permission?" .)