A J.P. most people have heard of (5)
I believe the answer is:
getty
'a j p most people' is the definition.
I can't tell whether this definition defines the answer.
'have heard of' is the wordplay.
'have' becomes 'get' (synonyms).
'heard of' becomes 'ty' (I can't justify this - if you can you should believe this answer much more).
'get'+'ty'='GETTY'
Can you help me to learn more?
(Other definitions for getty that I've seen before include "Billionaire art collector" , "J.Paul - - , museum donor" , "Malibu museum" , "John Paul - - , philanthropist" .)