Beam when Sarah turns up with leading lady (5)
I believe the answer is:
laser
'beam' is the definition.
(laser beam is a kind of beam)
'sarah turns up with leading lady' is the wordplay.
'sarah' becomes 'Sal' (Sally is a common pet form of Sarah).
'turns up' says the letters should be written backwards.
'with' is a charade indicator (letters next to each other).
'leading lady' becomes 'ER' (Elizabeth Regina).
'sal' written backwards gives 'las'.
'las'+'er'='LASER'
'when' is the link.
(Other definitions for laser that I've seen before include "Device producing a narrow, intense beam of light" , "scientific device" , "Device producing thin beam of light" , "E.g. surgical light-beam" , "Coherent light-beam" .)