A little brother's sitting up in bed, rapt (8)
I believe the answer is:
absorbed
'rapt' is the definition.
(synonyms)
'a little brother's sitting up in bed' is the wordplay.
'little brother's' becomes 'bro's'.
'sitting up' shows that the letters should be reversed in order (in a down clue, letters go up).
'in' is an insertion indicator.
'bros' back-to-front is 'sorb'.
'sorb' put inside 'bed' is 'bsorbed'.
'a'+'bsorbed'='ABSORBED'
(Other definitions for absorbed that I've seen before include "Was a sponge" , "Wholly engrossed or imbued" , "Engrossed, immersed" , "Deep in thought" , "Assimilated" .)