Incorporating cupped shape, extremely dull street lamps use this (6)
I believe the answer is:
sodium
'dull street lamps use this' is the definition.
The definition suggests a singular noun which matches the answer.
'incorporating cupped shape extremely' is the wordplay.
I cannot quite understand how this works, but
'extremely' could be 'so' (I've seen this in other clues) and 'so' is found in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for sodium that I've seen before include "Element in common salt" , "Atomic number 11" , "Alkali-metal element, Na" , "The Na in NaCl" , "Soft metallic element" .)