One adjusting the piano (by radio?) (5)
I believe the answer is:
tuner
'radio?' is the definition.
(synonyms)
'one adjusting the piano' is the wordplay.
I cannot really see how this works, but
'one' could be 'un' (I've seen this in other clues) and 'un' is located in the answer.
This may be the basis of clue (or it may be nonsense).
'by' acts as a link.
Can you help me to learn more?
(Other definitions for tuner that I've seen before include "piano man?" , "Piano adjuster" , "Radio receiver" , "Motor specialist" , "Radio adjuster" .)