Spots one charged with arson at last (4)
I believe the answer is:
Here is my best explanation:
'spots' is the definition.
(I've seen this before)
'one charged with arson at last' is the subsidiary indication.
'one charged' becomes 'ace' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'with' indicates putting letters inside.
'at last' says to take the final letters.
The last letter of 'arson' is 'n'.
'ace' going around 'n' is 'acne'.
(Other definitions for acne that I've seen before include "Teenage spots", "Teenage skin blemish", "Problem to face in", "Skin condition prevalent among teenagers", "Skin condition of adolescence", "Spots", "Adolescent spots", "Skin complaint", "(Inflammation causing) spots" and "Skin spots".)