Admission of fifty in party OK (10)
I believe the answer is:
disclosure
'admission' is the definition.
(thesaurus)
'fifty in party ok' is the wordplay.
'fifty' becomes 'L' (Roman numeral).
'in' indicates putting letters inside.
'party' becomes 'disco' (dance party).
'ok' becomes 'sure' (both can mean 'yes').
'l' going inside 'disco' is 'disclo'.
'disclo'+'sure'='DISCLOSURE'
'of' acts as a link.
(Other definitions for disclosure that I've seen before include "Publication" , "revealing" , "Release" , "Laying bare" , "leak" .)