Flashes extremely smart on dresses (7)
I believe the answer is:
strobes
'flashes' is the definition.
(I've seen this in another clue)
'extremely smart on dresses' is the wordplay.
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'on' says to put letters next to each other.
'dresses' becomes 'robes' (robing is a kind of dressing).
'smart' with its centre taken out is 'st'.
'st'+'robes'='STROBES'
(Other definitions for strobes that I've seen before include "Apparently freezes with light" , "Intermittently flashes" , "Flashy lights" .)