They say any element can be used for light (4)
I believe the answer is:
neon
'for light' is the definition.
(I've seen this in another clue)
'they say any element can be' is the wordplay.
'they say' shows a homophone (sound like).
'element' becomes 'O' (O is an example).
'can be' becomes 'n' (I can't explain this - if you can you should give a lot more credence to this answer).
'any' is a homophone of 'ne'.
'ne'+'o'+'n'='NEON'
'used' is the link.
Can you help me to learn more?
(Other definitions for neon that I've seen before include "Colourless, odourless rare gas" , "Noble gas used in illuminated signs" , "Gaseous element, in advertising sign say" , "Gaseous element, in advertising signs, say" , "Gas - light" .)