Light fittings, fifty SI base units (5)
I believe the answer is:
lamps
'light fittings' is the definition.
(I've seen this before)
'fifty si base units' is the wordplay.
I cannot quite understand how this works, but
'fifty' could be 'l' (Roman numeral) and 'l' is found in the answer.
'units' could be 'amps' (the amp is a unit of electrical current) and 'amps' is located in the answer.
No letters remain.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for lamps that I've seen before include "Lights" , "Devices for giving light" , "light fittings" .)