Topless dress to startle (4)
I believe the answer is:
rock
'startle' is the definition.
(I've seen this before)
'topless dress' is the wordplay.
'topless' means to remove the first letter.
'dress' becomes 'frock' (frocking is a kind of dressing).
'frock' with its initial letter taken away is 'ROCK'.
'to' acts as a link.
(Other definitions for rock that I've seen before include "solid?" , "Hard confectionery made in sticks" , "Shake; boulder" , "wobble" , "Sway - very big stone" .)