My acceleration due to gravity checked by device used when driving train (7)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

COR (cor! = my! = expressions of surprise), then G (g = symbol for the acceleration due to gravity) contained in (checked by) TEE (a device used to support a golf ball when a player is preparing to drive it). Cortege = train = a procession.