Plans get lost crossing border area (8)
I believe the answer is:
schemata
'plans' is the definition.
(schema is a kind of plan)
'get lost crossing border area' is the wordplay.
'get lost' becomes 'scat' (informal imperative meaning 'go away').
'crossing' is an insertion indicator.
'border' becomes 'hem' (I've seen this before).
'area' becomes 'a' (maths abbreviation).
'scat' enclosing 'hem' is 'schemat'.
'schemat'+'a'='SCHEMATA'
(Other definitions for schemata that I've seen before include "Diagrammatic outlines" , "Plans" , "diagrams" .)