Smart old Pence slammed paper (6)
I believe the answer is:
'smart' is the definition.
(I've seen this before)
'old pence slammed paper' is the wordplay.
'old pence' becomes 'd' (I can't justify this - if you can you should believe this answer much more).
'slammed' is an anagram indicator.
'paper' with letters rearranged gives 'apper'.
Can you help me to learn more?
(Other definitions for dapper that I've seen before include "Quick", "angler letting bait bob on water?", "Neat and well-dressed", "Smart in dress and manners", "Neat and smart in appearance".)