29ac Hikes are exhausting: do more training? (7). I have U - S - I - L.
Also 52ac Large circle, in the end twice as high (7). EQUATOR fits, and is a large circle, but I can't parse the wordplay....
(1a) When eighteen score degrees, we're back where we started. (4, 6) (14d) A small alteration that brings improvement - or some coins for a gambler? (1, 6, 3, 3, 6) (42d) Making that call in 2018?...
47A First for the First Major Slammer (4) g--e
74A American RacingSeries (4)Initials -,-,-,C
100A From the Banner to the Crokes (4) D-L-
56D The Number 1 Snub (7) c-u---n...