Additional mouth on the radio (4)
I believe the answer is:
more
'additional' is the definition.
(I know that additional can be written as more)
'mouth on the radio' is the wordplay.
I cannot quite understand how this works, but
'on' could be 're' (regarding) and 're' is present in the answer.
The remaining letters 'mo' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for more that I've seen before include "author" , "Extra (for Sir Thomas?)" , "Writer" , "A greater amount" , "Greater quantity or number" .)