Stranger not even seen by monarch (5)
I believe the answer is:
odder
'stranger' is the definition.
(I've seen this before)
'not even seen by monarch' is the wordplay.
'not even' becomes 'odd' ('odd' is the opposite to 'even'**).
'seen by' is a charade indicator (letters next to each other).
'monarch' becomes 'er' (abbreviation for Elizabeth Regina).
'odd'+'er'='ODDER'
(Other definitions for odder that I've seen before include "More unusual" , "More strange or less even" , "Stranger" , "More peculiar" , "more rum" .)