Blue Berets merited one story paper put in without reason (13)
I believe the answer is:
unjustifiable
'without reason' is the definition.
'blue berets merited one story paper put in' is the wordplay.
'blue berets' becomes 'un' (United Nations peacekeepers).
'merited one' becomes 'justi' (I can't explain this - if you can you should give a lot more credence to this answer).
'story' becomes 'fable' (fable is a kind of story).
'paper' becomes 'i' (UK newspaper).
'put in' indicates putting letters inside.
'fable' enclosing 'i' is 'fiable'.
'un'+'justi'+'fiable'='UNJUSTIFIABLE'
(Another definition for unjustifiable that I've seen is " Indefensible".)