Patch marks boundary (4)
I believe the answer is:
mend
'patch' is the definition.
('mend' can be a synonym of 'patch')
'marks boundary' is the wordplay.
'marks' becomes 'm' (former German currency).
'boundary' becomes 'end' (end is a kind of boundary).
'm'+'end'='MEND'
(Other definitions for mend that I've seen before include "'Restore, fix (4)'" , "Make improvements" , "Put right" , "Return to health; repair" , "Fix, repair" .)