Extremely short dress revealed by flashing light (6)
I believe the answer is:
strobe
'flashing light' is the definition.
(I've seen this before)
'extremely short dress' is the wordplay.
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'dress' becomes 'robe' (robing is a kind of dressing).
'short' with its middle taken out is 'st'.
'st'+'robe'='STROBE'
'revealed by' acts as a link.
(Other definitions for strobe that I've seen before include "Rapidly flashing lighting often used on stage" , "something flashy" , "Sort of lighting" , "Protestant reformer" , "Flashing-light device" .)