Hi there, Richard: Hamlet knew him (6)
I believe the answer is:
yorick
'hamlet knew him' is the definition.
('Alas, poor Yorick. I knew him' - Hamlet)
'hi there richard' is the wordplay.
'hi there' becomes 'yo' ('yo' is a greeting).
'richard' becomes 'rick' (common nickname for Richard).
'yo'+'rick'='YORICK'
(Other definitions for yorick that I've seen before include "Skull in 'Hamlet'" , "A fellow of infinite jest (Hamlet)" , "Well-known to Hamlet" , "White-face clown" , "Shakespeare character" .)