Dull smear around mirror's edge (4)
I believe the answer is:
drab
'dull' is the definition.
(synonyms)
'smear around mirror's edge' is the wordplay.
'smear' becomes 'dab' (I've seen this in another clue).
'around' is an insertion indicator.
'mirror's edge' becomes 'r' (I am not sure about this - if you are sure you should believe this answer much more).
'dab' enclosing 'r' is 'DRAB'.
Can you help me to learn more?
(Other definitions for drab that I've seen before include "Gloomy" , "Of a dull greenish-brown colour" , "Dull, uninteresting" , "Shabby; a slattern (arch.)" , "Dull and colourless" .)