Prince, so we hear, getting booster (6)
I believe the answer is:
fillip
'booster' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'prince so we hear' is the wordplay.
'prince' becomes 'Philip' (Prince Philip, the Duke of Edinburgh).
'so we hear' shows a homophone (sound like).
'philip' is a homophone of 'FILLIP'.
'getting' is the link.
Can you help me to learn more?
(Other definitions for fillip that I've seen before include "19, which sounds like a boy's name" , "give encouragement" , "shot in the arm" , "Stimulus, quick boost" , "If pill could be used as a stimulant" .)