Engrossed or captivated (4)
I believe the answer is:
rapt
I believe this is a double definition.
'engrossed' is the first definition.
(synonyms)
'captivated' is the second definition.
(I've seen this before)
(Other definitions for rapt that I've seen before include "Absorbed and enchanted" , "Ecstatic in part" , "Delighted" , "Fully engaged" , "Part of being intent and absorbed" .)