Material in publication on Muslim festival coming up (8)
I believe the answer is:
organdie
'material' is the definition.
(organdie is a kind of material)
'publication on muslim festival coming up' is the wordplay.
'publication' becomes 'organ' (organ is a kind of publication).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'muslim festival' becomes 'eid' (Islamic term for festival, eg Eid al-Fitr).
'coming up' shows that the letters should be reversed in order (in a down clue, letters go up).
'eid' back-to-front is 'die'.
'organ'+'die'='ORGANDIE'
'in' acts as a link.
(Other definitions for organdie that I've seen before include "Translucent cotton fabric, usually stiffened" , "Badly ignored a fine cotton muslin" , "Orange I'd changed for fine muslin" , "Oddly ignored a fine cotton muslin" , "Cotton fabric used for dresses" .)