Wholly engrossed, knocked sharply to be heard (4)
I believe the answer is:
rapt
'wholly engrossed' is the definition.
(I've seen this in another clue)
'knocked sharply to be heard' is the wordplay.
'knocked sharply' becomes 'rapped' (I've seen this in another clue).
'to be heard' shows a homophone (sound like).
'rapped' sounds like 'RAPT'.
(Other definitions for rapt that I've seen before include "Ecstatic in part" , "Engrossed, spellbound" , "Fully absorbed" , "carried away" , "Fully intent, fascinated" .)