Plays harp erratically, overshadowing those who are more on the ball (7)
I believe the answer is:
sharper
'are more on the ball' is the definition.
(I've seen this in another clue)
'plays harp erratically overshadowing' is the wordplay.
'overshadowing' says the answer is hidden in the clue (I've seen 'overshadows' mean this).
'SHARPER' is hidden within 'plays harp erratically'.
'those who' acts as a link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for sharper that I've seen before include "More acute" , "More acerbic" , "Card player" , "Swindler (at cards)" , "Cheat" .)