Initially Super Trouper dress is light … (6)
I believe the answer is:
strobe
'is light' is the definition.
(I've seen this before)
'initially super trouper dress' is the wordplay.
'initially' suggests taking the first letters.
'dress' becomes 'robe' (robing is a kind of dressing).
The initial letters of 'super trouper' is 'st'.
'st'+'robe'='STROBE'
(Other definitions for strobe that I've seen before include "Light that seems to spin" , "Flasher" , "Flashing lamp" , "something flashy" , "High-intensity flashing beam of light" .)