Friend sent back best computer (6)
I believe the answer is:
laptop
'computer' is the definition.
(laptop is a part of computer)
'friend sent back best' is the wordplay.
'friend' becomes 'pal' (pal can mean a friend).
'sent back' says the letters should be written backwards.
'best' becomes 'top' (top can mean the very best).
'pal' reversed gives 'lap'.
'lap'+'top'='LAPTOP'
(Other definitions for laptop that I've seen before include "Kind of computer" , "PC" , "Some work on this" , "Small computer" , "Aid for businessman" .)