Circuit best for computer (6)
I believe the answer is:
laptop
'computer' is the definition.
(laptop is a part of computer)
'circuit best' is the wordplay.
'circuit' becomes 'lap' (synonyms).
'best' becomes 'top' (top can mean the very best).
'lap'+'top'='LAPTOP'
'for' acts as a link.
(Other definitions for laptop that I've seen before include "PC" , "Portable computer" , "Some work on this" , "Type of computer" , "device?" .)