Observed European keeping queen calm (6)
I believe the answer is:
serene
'calm' is the definition.
(I know that calm can be written as serene)
'observed european keeping queen' is the wordplay.
'observed' becomes 'seen' (synonyms).
'european' becomes 'e' (abbreviation e.g. EU).
'keeping' means one lot of letters goes inside another (some letters kept inside).
'queen' becomes 'r' (abbreviation for regina, e.g. in ER ).
'seen'+'e'='seene'
'seene' going around 'r' is 'SERENE'.
(Other definitions for serene that I've seen before include "Impassive" , "Placid" , "Peaceful and unruffled" , "Calm, peaceful, tranquil" , "Not agitated" .)