Wash petticoat, then burn (10)
I believe the answer is:
slipstream
'wash' is the definition.
(synonyms)
'petticoat then burn' is the wordplay.
'petticoat' becomes 'slip' (I've seen this in another clue).
'then' means one lot of letters go next to another.
'burn' becomes 'stream' (burn is a Scottish dialect word for a stream).
'slip'+'stream'='SLIPSTREAM'
(Other definitions for slipstream that I've seen before include "Aerodynamic effect" , "Air current" , "Air driven back by a propeller" , "wake of moving vehicle?" , "Draught" .)