King careless about warning device (6)
I believe the answer is:
klaxon
'warning device' is the definition.
(klaxon is a kind of warning device)
'king careless about' is the wordplay.
'king' becomes 'k'.
'careless' becomes 'lax' ('lax' can be a synonym of 'careless').
'about' becomes 'on' (**).
'k'+'lax'+'on'='KLAXON'
(Other definitions for klaxon that I've seen before include "warning device" , "Old car horn" , "source of warning" , "Warning hooter" , "Blaring horn" .)