Astonish in topless dress (4)
I believe the answer is:
rock
'astonish' is the definition.
(thesaurus)
'topless dress' is the wordplay.
'topless' means to remove the first letter.
'dress' becomes 'frock' (a frock is a fancy dress).
'frock' with its initial letter taken away is 'ROCK'.
'in' acts as a link.
(Other definitions for rock that I've seen before include "Sweet < music" , "Brighton - (novel); - Hudson" , "Seaside sweet" , "Seaside confectionery" , "Hard confectionery made in sticks" .)