College backed up reserve computer (8)
I believe the answer is:
notebook
'computer' is the definition.
(type of portable computer)
'college backed up reserve' is the wordplay.
'college' becomes 'eton' (Eton College, English private school).
'backed up' is a reversal indicator.
'reserve' becomes 'book' (booking is a kind of reserving).
'eton' written backwards gives 'note'.
'note'+'book'='NOTEBOOK'
(Other definitions for notebook that I've seen before include "Token boo as in pocket recorder with blank pages (8)" , "What's associated with reporter" , "Small volume for brief records" , "Jotter - portable computer" , "Bigger than diary, smaller than laptop" .)