Laugh in front of the Queen! That's a warning (6)
I believe the answer is:
hooter
'that's a warning' is the definition.
(type of siren)
'laugh in front of the queen' is the wordplay.
'laugh' becomes 'hoot' (I've seen this before).
'in front of' is a charade indicator (letters next to each other).
'the queen' becomes 'er' (Elizabeth Regina).
'hoot'+'er'='HOOTER'
(Other definitions for hooter that I've seen before include "It makes a loud noise" , "Warning sound" , "One scents" , "Factory siren" , "alarm" .)