Turner starts to rise once Temeraire overshadows rivals (5)
I believe the answer is:
rotor
'turner' is the definition.
(something that turns)
'starts to rise once temeraire overshadows rivals' is the wordplay.
'starts to' indicates taking the first letters.
The initial letters of 'rise once temeraire overshadows rivals' is 'ROTOR'.
(Other definitions for rotor that I've seen before include "Hub with radiating blades" , "part of chopper" , "Helicopter propeller" , "Revolving dynamic part" , "Flywheel" .)