Radio Four's back on air (5)
I believe the answer is:
tuner
'radio' is the definition.
(I know that radio can be written as tuner)
'four's back on air' is the wordplay.
'back' says to take the final letters (the letter at the back).
'on' is a charade indicator (letters next to each other).
'air' becomes 'tune' (air can mean a piece of music).
The final letter of 'four' is 'r'.
'r' put after 'tune' is 'TUNER'.
(Other definitions for tuner that I've seen before include "Radio receiver" , "Radio adjuster" , "Motor specialist" , "piano man?" , "Piano adjuster" .)