College sent back reserve computer (8)
I believe the answer is:
notebook
'computer' is the definition.
(I've seen this before)
'college sent back reserve' is the wordplay.
'college' becomes 'eton' (I've seen this before).
'sent back' says the letters should be written backwards.
'reserve' becomes 'book' (synonyms).
'eton' backwards is 'note'.
'note'+'book'='NOTEBOOK'
(Other definitions for notebook that I've seen before include "Jotter - portable computer" , "Token boo as in pocket recorder with blank pages (8)" , "What's associated with reporter" , "jotting resource" , "little computer" .)