Spoils a coat tail with a nasty stain (6)
I believe the answer is:
taints
'spoils' is the definition.
(tainting is a kind of spoiling)
'a coat tail with a nasty stain' is the wordplay.
'tail' suggests the final letters.
'with' says to put letters next to each other.
'a nasty' indicates anagramming the letters.
The last letter of 'coat' is 't'.
'stain' anagrammed gives 'aints'.
't'+'aints'='TAINTS'
(Other definitions for taints that I've seen before include "Traces of decay in Titans" , "Flaws in titans" , "Traces of bad quality in titans" , "Contaminates" , "Pollutes" .)