Most tiny powerless lights put back in case (8)
I believe the answer is:
smallest
'most tiny' is the definition.
('small' can be similar in meaning to 'tiny')
'powerless lights put back in case' is the wordplay.
'powerless lights' becomes 'lams' (I can't justify this - if you can you should believe this answer much more).
'put back' says the letters should be written in reverse.
'in case' becomes 'lest' (eg 'lest we forget').
'lams' reversed gives 'smal'.
'smal'+'lest'='SMALLEST'
Can you help me to learn more?
(Other definitions for smallest that I've seen before include "Most diminutive" , "Least in size or quantity" , "Most ungenerous" , "Most dimunitive" , "Tiniest" .)