Opening foreign letter with edge of thumb makes bad mark (6)
I believe the answer is:
stigma
'bad mark' is the definition.
(stigma is the mark or taint of something bad)
'opening foreign letter with edge of thumb' is the wordplay.
'opening' means one lot of letters goes inside another (some letters open a gap in others).
'foreign letter' becomes 'sigma' (Greek letter).
'with edge of thumb' becomes 't' (I can't explain this - if you can you should believe this answer much more).
'sigma' enclosing 't' is 'STIGMA'.
'makes' acts as a link.
(Other definitions for stigma that I've seen before include "Badge of shame" , "Disgraceful mark" , "Bad mark" , "part of plant" , "Brand or mark of infamy" .)