Sharp quality, putting quiche on head (8)
I believe the answer is:
tartness
'sharp quality' is the definition.
(I know this)
'quiche on head' is the wordplay.
'quiche' becomes 'tart'.
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'head' becomes 'ness' (both can mean a promontory).
'tart'+'ness'='TARTNESS'
'putting' acts as a link.
(Other definitions for tartness that I've seen before include "pungency" , "bitter taste" .)