European monarch twice hosts one stranger (6)
I believe the answer is:
eerier
'stranger' is the definition.
(eerie means odd or strange)
'european monarch twice hosts one' is the wordplay.
'european' becomes 'E' (abbreviation e.g. EU).
'monarch' becomes 'er' (abbreviation for Elizabeth Regina).
'twice' shows that letters should be duplicated.
'hosts' is an insertion indicator (inserted letters are welcomed or hosted).
'one' becomes 'i' (Roman numeral).
'er' duplicated is 'erer'.
'erer' going around 'i' is 'erier'.
'e'+'erier'='EERIER'
(Other definitions for eerier that I've seen before include "More mysterious" , "Weirder" , "Spookier" , "More unsettling" , "More uncanny" .)