A measure of light tin and lead compound (7)
I believe the answer is:
candela
'a measure of light' is the definition.
(I've seen this before)
'tin and lead compound' is the wordplay.
'tin' becomes 'can' (synonyms).
'and' says to put letters next to each other.
'compound' is an anagram indicator.
'lead' is an anagram of 'dela'.
'can'+'dela'='CANDELA'
(Other definitions for candela that I've seen before include "measure of light" , "Measure of brilliance" , "Unit of luminous intensity" .)