Heath upended board to go ashore (8)
I believe the answer is:
'heath' is the definition.
(I've seen this before)
'upended board to go ashore' is the wordplay.
'upended' says the letters should be written backwards (in a down clue, letters go up).
'board' becomes 'room' ('room' can be a synonym of 'board').
'to go ashore' becomes 'land'.
'room' back-to-front is 'moor'.
(Other definitions for moorland that I've seen before include "Area of uncultivated hills" , "Extensive upland tract" , "Large uncultivated area" , "Open, uncultivated tract" , "Tract of open shrubby ground" .)