Plain cloth covered in mystical icons (6)
I believe the answer is:
calico
'plain cloth' is the definition.
'calico' can be an answer for 'cloth' (calico is a kind of cloth). I'm unsure of the 'plain' bit.
'covered in mystical icons' is the wordplay.
'covered in' says the answer is hidden in the clue.
'CALICO' is hidden in the letters of 'mystical icons'.
Can you help me to learn more?
(Other definitions for calico that I've seen before include "Coarse printed cotton cloth" , "Unbleached sixteen down cloth" , "Type of cotton cloth" , "Cotton cloth or fabric" , "Coarse cloth with a bright print" .)