Authorisation to turn clock back (5)
I believe the answer is:
remit
'authorisation to turn' is the definition.
I can't judge whether this definition defines the answer.
'clock back' is the wordplay.
'clock' becomes 'timer' (something which times).
'back' shows that the letters should be reversed in order.
'timer' backwards is 'REMIT'.
Can you help me to learn more?
(Other definitions for remit that I've seen before include "Relax; send (payment)" , "instructions required" , "Send (money) by post" , "Scope - send money" , "Give up" .)