Girl hosting old monarch beams (6)
I believe the answer is:
lasers
'beams' is the definition.
(I've seen this in another clue)
'girl hosting old monarch' is the wordplay.
'girl' becomes 'lass' (lass can informally mean a girl).
'hosting' means one lot of letters goes inside another (inserted letters are welcomed in or hosted).
'old monarch' becomes 'ER' (Elizabeth Regina, ie Queen Elizabeth).
'lass' going around 'er' is 'LASERS'.
(Other definitions for lasers that I've seen before include "Optical devices" , "Light beam devices" , "Light amplifications by stimulated emissions of radiation" , "Intense beams of light" .)