Blue Berets are guarding Washington in the dark (7)
I believe the answer is:
unaware
'in the dark' is the definition.
(I've seen this before)
'blue berets are guarding washington' is the wordplay.
'blue berets' becomes 'UN' (United Nations peacekeepers).
'guarding' is an insertion indicator.
'washington' becomes 'wa' (two letter abbreviation for the state).
'are' placed around 'wa' is 'aware'.
'un'+'aware'='UNAWARE'
(Other definitions for unaware that I've seen before include "Not knowing (of a situation or fact)" , "Unexpected" , "Unwitting, not knowing" , "Not cognisant" , "Nescient" .)