Board bound up with new addition (5)
I believe the answer is:
panel
'board' is the definition.
(I know that board can be written as panel)
'bound up with new addition' is the wordplay.
'bound' becomes 'leap' (synonyms).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'with' means one lot of letters goes inside another.
'new addition' becomes 'n' (I can't justify this - if you can you should believe this answer much more).
'leap' in reverse letter order is 'pael'.
'pael' placed around 'n' is 'PANEL'.
Can you help me to learn more?
(Other definitions for panel that I've seen before include "Section of wood or metal" , "Wall mounting" , "Body section" , "Group of judges" , "Rect. section; jury" .)