In can on top of mantelpiece, mixed spice (8)
I believe the answer is:
cinnamon
'spice' is the definition.
(cinnamon is a kind of spice)
'in can on top of mantelpiece mixed' is the wordplay.
'top of' suggests taking the first letters.
'mixed' is an anagram indicator.
The first letter of 'mantelpiece' is 'm'.
'in'+'can'+'on'+'m'='incanonm'
'incanonm' anagrammed gives 'CINNAMON'.
(Other definitions for cinnamon that I've seen before include "Yellow-brown spice from bark of Asian tree" , "Yellowish-brown - spice" , "'Spice in con man, strangely (8)'" , "Spicy bark" , "spicy stuff" .)