Turner starts on representation of rocky hill (5)
I believe the answer is:
rotor
'turner' is the definition.
(something that turns)
'starts on representation of rocky hill' is the wordplay.
'starts on' indicates taking the first letters.
'rocky hill' becomes 'tor' (I've seen this in another clue).
The first letters of 'representation of' is 'ro'.
'ro'+'tor'='ROTOR'
(Other definitions for rotor that I've seen before include "Machine part that turns around" , "Mechanical turning part" , "Turning part of machine - a palindrome" , "Turning part of a mechanism" , "Rotating part of machine" .)