Second-in-command

The Prince of Orange served as second-in-command of the Allied army under Arthur Wellesley, Duke of Wellington. (PROSE: World Game)

Daleks
In the original Dalek hierarchy, the Dalek Prime or Emperor Dalek's immediate subordinate was the Black Dalek, also known as the Dalek Supreme, who served as the warlord of the Dalek race. (PROSE: The History of the Daleks)

During the 2150s Dalek invasion of Earth, the Black Supreme Controller was supported by a second-in-command. (PROSE: Doctor Who and the Dalek Invasion of Earth)

The gold-and-black Dalek Supreme who oversaw the Spiridon campaign in 2540 was second to the Dalek Emperor, serving as head of the Dalek Supreme Council. (PROSE: Doctor Who and the Planet of the Daleks)

During the Last Great Time War, the Dalek Time Strategist had the coveted rank of second-in-command of the Dalek Empire, serving as an advisor to the Dalek Emperor. (AUDIO: Restoration of the Daleks, Temmosus)

Following the establishment of the Parliament of the Daleks, the original Supreme Dalek of the New Dalek Paradigm served as second-in-command to the Prime Minister of the Daleks, (TV: Asylum of the Daleks, PROSE: The Secret Lives of Monsters) who superseded the position of Dalek Emperor. (PROSE: Dalek Combat Training Manual) This form of the Dalek hierarchy stood for centuries until the Supreme, during the Siege of Trenzalore, exterminated the Prime Minister and took command of the Parliament before they were themselves wiped out by the Eleventh Doctor's regeneration. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe)

Sontarans
Sontaran Commander Skorr was second-in-command for General Staal of the Tenth Sontaran Battle Fleet. (TV: The Sontaran Stratagem|The Poison Sky)

Sontaran Colonel Sarg was second-in-command for General Kade of the Fourth Sontaran Intelligence Division, up to when he was slain in a fight to the death for rebellion. (PROSE: The Taking of Chelsea 426)

Sontaran Captain Skeed was second-in-command for Major Stenx of the Twelfth Sontaran Battle Fleet. (PROSE: The Sontaran Games)

Behind the scenes

 * The Doctor Who Technical Manual (1983) makes the claim that the positions of Dalek Supreme and Black Dalek are a synonymous second-in-command to the Emperor Dalek, and that thus there was only ever one Black Supreme at any given point in Dalek history.