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'm not sure about the 'american' bit.
'most of border invaded' is the wordplay.
'most' becomes 'bra' (I am not sure about this - if you are sure you should believe this answer much more).
'of border' says to hollow out the word (remove centre letters) (I've seen 'borders' mean this).
'invaded' becomes 'intruded on' ('intrude on' can be a synonym of 'invade').
'intrudedon' with its middle taken out is 'in'.
'bra'+'in'='BRAIN'
'by' is the 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" .)