Remnant of a letter to artless Stuart (4)
I believe the answer is:
stub
'remnant' is the definition.
(I've seen this in another clue)
'letter to artless stuart' is the wordplay.
'letter' becomes 's'.
'to artless stuart' becomes 'tub' (I can't justify this - if you can you should give a lot more credence to this answer).
's'+'tub'='STUB'
'of a' acts as a link.
Can you help me to learn more?
(Other definitions for stub that I've seen before include "Counterfoil, of a cheque say" , "this pollution by smoker?" , "Put out" , "End of cigarette or counterfoil of cheque" , "seen in the ash-tray" .)