Brightness shown by one in uni, mostly working (10)

'brightness' is the definition.
(I know that brightness can be written as luminosity)

'one in uni mostly working' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' means one lot of letters goes inside another.
'working' indicates anagramming the letters.
'uni'+'mostly'='unimostly'
'unimostly' is an anagram of 'luminosty'.
'i' put within 'luminosty' is 'LUMINOSITY'.

'shown by' is the link.