Flasher starts to swish their gown (6)
I believe the answer is:
strobe
'flasher' is the definition.
(I've seen this before)
'starts to swish their gown' is the wordplay.
'starts to' suggests taking the first letters.
'gown' becomes 'robe' (synonyms).
The initial letters of 'swish their' is 'st'.
'st'+'robe'='STROBE'
(Other definitions for strobe that I've seen before include "Protestant reformer" , "Rapidly flashing lighting often used on stage" , "Flasher" , "Flashing lamp" , "Flash on and off" .)