Free concert largely by rock singer (3,4)
I believe the answer is:
pro bono
'free' is the definition.
(I've seen this in another clue)
'concert largely by rock singer' is the wordplay.
'concert' becomes 'prom' (abbreviation for promenade concert).
'largely' means to remove the last letter.
'by rock singer' becomes 'bono' (I can't explain this - if you can you should give a lot more credence to this answer).
'prom' with its last letter taken away is 'pro'.
'pro'+'bono'='PRO BONO'
Can you help me to learn more?
(Other definitions for pro bono that I've seen before include "For the public good - ... .... publico" , "not being paid" , "Giving free services" , "Done for the public good unpaid" , "with no charges" .)