Posh school transforming its fourth famous actress (6)
I believe the answer is:
harlow
'famous actress' is the definition.
(Jean Harlow)
I don't understand how the remainder of the clue works.
(Other definitions for harlow that I've seen before include "Essex town - American film actress, d. 1937" , "place in Essex" , "Essex girl?" , "Blonde bombshell" , "FILMSTAR" .)