50a. Each half or quarter of a folding backgammon board, a slab inscribed with laws, or a matrix of data arranged in columns and rows. (5)
54a. Item of furniture with arms, legs, a back and sometimes wings. (5)
55a. Mineral mass forming part of the Earth’s crust such as chalk, granite, marble or limestone (4)
1d. Brief cascade fall or flight of something such as arrows, raindrops, shooting stars, snowflakes, sparks or subatomic particles in the form of cosmic rays etc. (4)