Kind of light garment under servant's coat (6)
I believe the answer is:
strobe
'kind of light' is the definition.
(I've seen this in another clue)
'garment under servant's coat' is the wordplay.
'garment' becomes 'robe' (robe is a type of garment).
'under' means one lot of letters go next to another.
'coat' suggests removing the centre (only the letters which coat the word are used).
'servant' with its centre removed is 'st'.
'robe' put after 'st' is 'STROBE'.
(Other definitions for strobe that I've seen before include "Flashing disco light" , "High-intensity flashing beam of light" , "Sorbet (anag) -flashing light" , "Sister to Hansel" , "Flashing-light device" .)