Mug seen in dark is serious (6)
I believe the answer is:
kisser
'mug' is the definition.
(I know that mug can be written as kisser)
'in dark is serious' is the wordplay.
'in' indicates the answer is hidden within the clue.
'KISSER' is hidden in the letters of 'dark is serious'.
'seen' acts as a link.
(Other definitions for kisser that I've seen before include "Amorous type" , "Slangy face of osculatory origin" , "Skiers (anag.)" , "Mouth (sl.)" , "Casanova" .)