You must put different questions on radio or else! (9)
I believe the answer is:
otherwise
'or else' is the definition.
(I've seen this before)
'you must put different questions on radio' is the wordplay.
I cannot really understand how this works, but
'put' could be 'the' (I've seen this in other clues) and 'the' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for otherwise that I've seen before include "White rose seen in a different way" , "Under different conditions" , "Alternatively" , "in different circumstances" , "Apart from that" .)