Gas is primarily poor conductor (7)
I believe the answer is:
prattle
'gas' is the definition.
(thesaurus)
'primarily poor conductor' is the wordplay.
'primarily' says to take the initial letters.
'conductor' becomes 'rattle' (Rattle is an orchestra conductor).
The first letter of 'poor' is 'p'.
'p'+'rattle'='PRATTLE'
'is' acts as a link.
(Other definitions for prattle that I've seen before include "Idle chatter" , "Chatter inconsequentially, natter" , "Idle talk" , "Be vapidly garrulous" , "Foolish talk" .)