Plank (4)
(Other definitions for spar that I've seen before include "Pole to support sails, rigging etc" , "supporting member" , "microcrystalline mineral?" , "Box gently; main beam of aircraft wing" , "Act as boxer" .)
'plank' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for board that I've seen before include "Collection of directors" , "See 9" , "Decision-making body" , "See 21 Down" , "Plank" .)
(Other definitions for shelf that I've seen before include "grass down under" , "sort of life" , "Ledge for books etc" , "Ledge on which to stand items" , "support for books?" .)