Bob reads rubbish when engrossed (8)
I believe the answer is:
absorbed
'engrossed' is the definition.
('absorb' can be a synonym of 'engross')
'bob reads rubbish' is the wordplay.
'rubbish' indicates anagramming the letters (a poor quality or rubbish spelling of the letters).
'bob'+'reads'='bobreads'
'bobreads' with letters rearranged gives 'ABSORBED'.
'when' is the link.
(Other definitions for absorbed that I've seen before include "Rapt, intent" , "Wholly engrossed or imbued" , "Engrossed, immersed" , "Assimilated" , "Was a sponge" .)