Monarch to speak wildly — that’s out of place (6)
I believe the answer is:
errant
'out of place' is the definition.
'monarch to speak wildly' is the wordplay.
'monarch' becomes 'er' (abbreviation for Elizabeth Regina).
'to speak wildly' becomes 'rant'.
'er'+'rant'='ERRANT'
'that's' acts as a link.
(Other definitions for errant that I've seen before include "Misbehaving" , "Sinful" , "Travelling about or going astray" , "looking for adventure" , "so some knights are called" .)