King not strict enough on loud horn (6)
I believe the answer is:
klaxon
'loud horn' is the definition.
(I've seen this in another clue)
'king not strict enough on' is the wordplay.
'king' becomes 'K' (chess abbreviation).
'not strict enough' becomes 'lax' (I've seen this before).
'k'+'lax'+'on'='KLAXON'
(Other definitions for klaxon that I've seen before include "A kind of loud horn" , "source of warning" , "warning device" , "Old car horn" , "Warning horn" .)