Totally engrossed (4)
I believe the answer is:
rapt
'totally engrossed' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for rapt that I've seen before include "riveted" , "Engrossed, spellbound" , "Taken up entirely" , "Absorbed and enchanted" , "'Transported, entranced (4)'" .)