second-in-command

plural seconds-in-command

1. Somebody ranking next below a commander.
2. Someone ranking next below the person in charge.

   Allan was first William Buddicom's and then Trevithick's second-in-command, at Edge Hill and at Crewe successively, for the period of 13½ years from February, 1840, to September, 1853, when he left Crewe to become Superintendent of the Scottish Central Railway.

   1951 June, Dr. W. H. Chaloner, “Alexander Allan's Own Claim”, in Railway Magazine, page 416

   Synonyms: number two