Most of border invaded by American intelligence (5)
I believe the answer is:
brain
'american intelligence' is the definition.
'brain' can be an answer for 'intelligence' (brain is a kind of intelligence). I am not sure about the 'american' bit.
'most of border invaded' is the wordplay.
'most' becomes 'bra' (I can't justify this - if you can you should believe this answer much more).
'of border' means to remove the middle letters (I've seen 'borders of' mean this).
'invaded' becomes 'intruded on' ('intrude on' can be a synonym of 'invade').
'intrudedon' with its centre taken out is 'in'.
'bra'+'in'='BRAIN'
'by' acts as a link.
Can you help me to learn more?
(Other definitions for brain that I've seen before include "and 15 ac. The loss of experts through emigration" , "Clever type" , "Whack on the head" , "Intelligence" , "Hit hard; intellect" .)