Stupefied by silver tile over top of skull (6)
I believe the answer is:
aghast
'stupefied' is the definition.
(I've seen this before)
'silver tile over top of skull' is the wordplay.
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
'tile' becomes 'hat' (tile is an old-fashioned term for a hat).
'over' means one lot of letters goes inside another.
'top of' indicates taking the first letters.
The initial letter of 'skull' is 's'.
'hat' placed around 's' is 'hast'.
'ag'+'hast'='AGHAST'
'by' acts as a link.
(Other definitions for aghast that I've seen before include "Filled with horror or shock" , "Amazed" , "Filled with fear or horror" , "Stupefied with horror" , "Shocked, dumbfounded" .)