Structural supports (6)
'structural supports' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for beams that I've seen before include "Smiles; joists" , "Rays of light" , "Shines" , "Smiles broadly or sunnily" .)
(Other definitions for joists that I've seen before include "Supporting beams in a floor or ceiling" , "Beams for floor or roof" .)