Beam when admitted by the French Queen (5)
I believe the answer is:
laser
'beam' is the definition.
(laser beam is a kind of beam)
'when admitted by the french queen' is the wordplay.
'when' becomes 'as' ().
'admitted by' means one lot of letters goes inside another.
'the french' becomes 'le' ('the' in French).
'queen' becomes 'r' (r is abbreviation regina. e.g. in ER ).
'as' put into 'le' is 'lase'.
'lase'+'r'='LASER'
(Other definitions for laser that I've seen before include "Source of light; type of printer" , "Scientific apparatus" , "Device yielding powerful beam" , "type of surgery" , "Device that generates an intense ray of light" .)