Until engineer has a slate (4)
I believe the answer is:
tile
'a slate' is the definition.
(thesaurus)
'until engineer has' is the wordplay.
'has' indicates a hidden word.
'TILE' is hidden within 'until engineer'.
(Other definitions for tile that I've seen before include "Small slab of baked clay for covering walls, roofs etc" , "Ceramic slab for roofs, walls, floors etc" , "Hat" , "Oblong piece of glazed clay for bathroom wall" , "Roof slabs" .)