Britain managed to implicate one intelligence agent (5)
I believe the answer is:
brain
'intelligence agent' is the definition.
'britain managed to implicate one' is the wordplay.
'britain' becomes 'b' (common abbreviation as in GB).
'managed' becomes 'ran' (I've seen this before**).
'to implicate' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'ran' going around 'i' is 'rain'.
'b'+'rain'='BRAIN'
(Other definitions for brain that I've seen before include "Clever person" , "Thinking organ" , "Clever type" , "The grey matter" , "Intelligence" .)