Filth, say, rising across border (5)
I believe the answer is:
grime
'filth' is the definition.
(I know that filth can be written as grime)
'say rising across border' is the wordplay.
'say' becomes 'eg' (both can mean 'for example').
'rising' says the letters should be written backwards (going up in a down clue).
'across' indicates putting letters inside.
'border' becomes 'rim' (I've seen this before).
'eg' in reverse letter order is 'ge'.
'ge' enclosing 'rim' is 'GRIME'.
(Other definitions for grime that I've seen before include "an ingrained phenomenon" , "Dirt, grease" , "Muck" , "Engrained dirt" , "Filth, dirt" .)