Sugary coating unwelcome on aircraft (5)
I believe the answer is:
icing
I believe this clue is a double definition.
'sugary coating' is the first definition.
(sugary decoration)
'unwelcome on aircraft' is the second definition.
(icing is a problem for an aircraft)
(Other definitions for icing that I've seen before include "Topping in a bag" , "Sweet thing" , "Embellishing the cake" , "Freezing" , "Showing effect of cold snap" .)