Last of sugar put into unhealthy-looking tart (6)
I believe the answer is:
pastry
'tart' is the definition.
(I've seen this in another clue)
'last of sugar put into unhealthy-looking' is the wordplay.
'last of' says to take the final letters.
'put into' means one lot of letters goes inside another.
'unhealthy-looking' becomes 'pasty' (pasty can mean looking pale).
The last letter of 'sugar' is 'r'.
'r' put inside 'pasty' is 'PASTRY'.
(Other definitions for pastry that I've seen before include "Cake - tart" , "croissant?" , "Piecrust; small cake" , "'Dough of flour, fat and water (6)'" , "Sweet baked dough" .)