Adjust, as a radio (4)
I believe the answer is:
tune
'adjust as a radio' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tune that I've seen before include "Adjust a radio receiver to a frequency" , "Improve quality" , "Melody; adjust (engine)" , "'He who pays the piper calls the ...'" , "Air, melody" .)