Green’s Dictionary of Slang

1. a thief who specializes in robbing houses while their occupants are out, usu. for a short time.

1934		J. Franklyn This Gutter Life 52: ‘A “drummer”,’ answered Gerrard. ‘Meaning?’ ‘She gets jobs as skivvy in a big house, takes plans, and some months later her man does the job; or on smaller occasions simply goes knocking at the door, to make sure the family are really out.’.
1941		V. Davis Phenomena in Crime 210: The ‘drummer’ [...] specializes in ‘dead’ houses, those having no occupant at the time of his visit.
1959		‘Screwsman’s Lament’ in Encounter n.d. in Norman Norman’s London (1969) 67: I was sitting in the boozer with my china, Drummer Bill, / And as it was his nature, he had his minces on the till.
1999		Guardian G2 17 Aug. 13: Ray, a house-breaker (or drummer) who has lately done three years in jail.
2004		N. ‘Razor’ Smith A Few Kind Words and a Loaded Gun 39: Couple of naughty little drummers.

2. (UK Und.) a thief’s assistant.

1959

J. Gosling Ghost Squad 150: He sent with him a ‘drummer’ — a man who had to make sure that the coast was clear and help the screwsman with unskilled jobs, such as carrying the ladder.