Forbidding end to obscene filth (5)
I believe the answer is:
grime
'filth' is the definition.
(I know that filth can be written as grime)
'forbidding end to obscene' is the wordplay.
'forbidding' becomes 'grim' (I've seen this in another clue).
'end to' says to take the final letters.
The final letter of 'obscene' is 'e'.
'grim'+'e'='GRIME'
(Other definitions for grime that I've seen before include "an ingrained phenomenon" , "Muck" , "Ingrained dirt" , "Soot" , "Engrained dirt" .)