Church on marshland boundary (5)
I believe the answer is:
fence
'boundary' is the definition.
(thesaurus)
'church on marshland' is the wordplay.
'church' becomes 'CE' (abbreviation for Church of England).
'on' means one lot of letters go next to another.
'marshland' becomes 'fen ' (fen is a type of wetland).
'ce' after 'fen' is 'FENCE'.
(Other definitions for fence that I've seen before include "feature in horse-racing?" , "who deals in hot stuff" , "Obstacle to clear, in steeple-chasing" , "Shield" , "Paling; criminal receiver" .)