Plan southern border in church (6)
I believe the answer is:
scheme
'plan' is the definition.
(scheme is a kind of plan)
'southern border in church' is the wordplay.
'southern' becomes 's' (abbreviation).
'border' becomes 'hem' (I've seen this in another clue).
'in' means one lot of letters goes inside another.
'church' becomes 'CE' (abbreviation for Church of England).
'hem' placed into 'ce' is 'cheme'.
's'+'cheme'='SCHEME'
(Other definitions for scheme that I've seen before include "Plot, plan secretly" , "Plan or act secretly" , "proposal" , "Plan, ruse" , "Plan; system" .)