Piece of equipment containing minimum of light is dark (5)
I believe the answer is:
unlit
'is dark' is the definition.
(similar in meaning)
'piece of equipment containing minimum of light' is the wordplay.
'piece of equipment' becomes 'unit' (I can't explain this - if you can you should believe this answer much more).
'containing' is an insertion indicator.
'minimum of' indicates taking the first letters.
The first letter of 'light' is 'l'.
'unit' placed around 'l' is 'UNLIT'.
Can you help me to learn more?
(Other definitions for unlit that I've seen before include "Shadowy" , "Shady" , "Perhaps invisible" , "Dark, gloomy" , "not provided with a lamp?" .)