Festivity at number one -- turned up for unveiling (10)
I believe the answer is:
revelation
'unveiling' is the definition.
(thesaurus)
'festivity at number one turned up' is the wordplay.
'festivity' becomes 'revel' (revel is a kind of festivity).
'number' becomes 'no' (abbreviation).
'one' becomes 'I' (Roman numeral).
'turned up' is a reversal indicator.
'no'+'i'='noi'
'noi' back-to-front is 'ion'.
'revel'+'at'+'ion'='REVELATION'
'for' is the link.
(Other definitions for revelation that I've seen before include "telling statement" , "Part of Bible" , "enlightening experience" , "Suprising disclosure" , "The last book to give clergyman" .)