Bulk of research by Lennon's partner is voluntary (3,4)
I believe the answer is:
pro bono
'voluntary' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.
'bulk of research by lennon's partner' is the wordplay.
'bulk of research' becomes 'prob' (I can't explain this - if you can you should believe this answer much more).
'by' is a charade indicator (letters next to each other).
'lennon's partner' becomes 'ono' (I've seen this in another clue).
'prob'+'ono'='PRO BONO'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for pro bono that I've seen before include "Giving free services" , "with no charges" , "Done for the public good unpaid" , "For the public good - ... .... publico" , "not being paid" .)