Computer needing circuit: the best (6)
I believe the answer is:
laptop
'computer' is the definition.
(laptop computer is a kind of computer)
'circuit the best' is the wordplay.
'circuit' becomes 'lap' ('lap' can be a synonym of 'circuit').
'the best' becomes 'top' (I've seen this before).
'lap'+'top'='LAPTOP'
'needing' acts as a link.
(Other definitions for laptop that I've seen before include "Some work on this" , "Kind of computer" , "Small computer" , "PC" , "device?" .)