Circuit best for computer (6)
I believe the answer is:
laptop
'computer' is the definition.
(laptop computer is a kind of computer)
'circuit best' is the wordplay.
'circuit' becomes 'lap' ('lap' can be a synonym of 'circuit').
'best' becomes 'top' ('top' can be similar in meaning to '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?" .)