One offering support to the board? (7)
I believe the answer is:
trestle
'one offering support' is the definition.
'trestle' can be an answer for 'support' (I've seen this before). I'm unsure of the 'one offering' bit.
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for trestle that I've seen before include "Letters (anag) - table" , "Simple table support" , "support for board" , "folding table?" , "Table support frame" .)