Blue Berets loved defending northern plain (9)
I believe the answer is:
unadorned
'plain' is the definition.
('unadorned' can be similar in meaning to 'plain')
'blue berets loved defending northern' is the wordplay.
'blue berets' becomes 'un' (United Nations peacekeepers).
'loved' becomes 'adored' ('adored' can be similar in meaning to 'loved').
'defending' indicates putting letters inside.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'adored' enclosing 'n' is 'adorned'.
'un'+'adorned'='UNADORNED'
(Other definitions for unadorned that I've seen before include "Simple and plain as around den" , "Without embellishment" , "Not decorated, plain and simple" , "Plain, undecorated" .)