Stone on dress flashing in light (6)
I believe the answer is:
strobe
'flashing in light' is the definition.
'strobe' can be an answer for 'light' (strobe light is a type of light). I'm not sure about the 'flashing in' bit.
'stone on dress' is the wordplay.
'stone' becomes 'st' (abbreviation).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'dress' becomes 'robe' (robing is a kind of dressing).
'st'+'robe'='STROBE'
Can you help me to learn more?
(Other definitions for strobe that I've seen before include "High-intensity flashing beam of light" , "Flashing, very bright beam" , "Protestant reformer" , "Flasher" , "Flashing-light device" .)