Hush-hush online fact I’d worked out (12)
I believe the answer is:
confidential
'hush-hush' is the definition.
(thesaurus)
'online fact i'd worked out' is the wordplay.
'worked out' indicates an anagram.
'online'+'fact'+'id'='onlinefactid'
'onlinefactid' with letters rearranged gives 'CONFIDENTIAL'.
(Other definitions for confidential that I've seen before include "Intended to be kept secret" , "Classified" , "Like T.S. Eliot's clerk" , "not expecting publication" , "Graham Greene's agent" .)