Evil spirit in marshland upset the old (5)
I believe the answer is:
bogey
'evil spirit' is the definition.
(bogey is a kind of evil spirit)
'marshland upset the old' is the wordplay.
'marshland' becomes 'bog' (piece of marshland).
'upset' shows that the letters should be reversed in order.
'the old' becomes 'ye' (resembles a historical spelling of 'the').
'ye' in reverse letter order is 'ey'.
'bog'+'ey'='BOGEY'
'in' acts as a link.
(Other definitions for bogey that I've seen before include "Evil spirit - golf score of one over par for a hole" , "One over in 20 across" , "Two over par for a hole" .)