Allowed to phone back? (3)
I believe the answer is:
let
'allowed to' is the definition.
(to let something happen is to allow it)
'phone back?' is the wordplay.
'phone' becomes 'tel'.
'back?' says the letters should be written backwards (I've seen 'back' mean this).
'tel' backwards is 'LET'.
(Other definitions for let that I've seen before include "Permitted" , "Allowed or rented out" , "Allow - hire" , "Won't stop you" , "Allow; permit" .)