He has something to say on the radio (7)
I believe the answer is:
speaker
'he' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'something to say on the radio' is the wordplay.
I cannot quite see how this works, but
'something' could be 'er' (I've seen this in other clues) and 'er' is found in the answer.
'say' could be 'speak' (synonyms) and 'speak' is found in the answer.
No letters remain.
This explanation may well be incorrect...
'has' is the link.
Can you help me to learn more?
(Other definitions for speaker that I've seen before include "Person presiding over British House of Commons or similar assembly" , "one who restores order" , "Lecturer" , "He asks for order" , "Commons presiding officer" .)