Spots one charged with arson at last (4)
I believe the answer is:
acne
'spots' is the definition.
(I've seen this before)
'one charged with arson at last' is the wordplay.
'one charged' becomes 'ace' (I can't explain this - if you can you should give a lot more credence to this answer).
'with' is an insertion indicator.
'at last' suggests the final letters.
The final letter of 'arson' is 'n'.
'ace' placed around 'n' is 'ACNE'.
Can you help me to learn more?
(Other definitions for acne that I've seen before include "Skin condition prevalent among teenagers" , "Adolescent spots" , "(Inflammation causing) spots" , "Teenage skin blemish" , "sort of disease" .)