One concealing fancy undies, mostly, in public (8)
I believe the answer is:
audience
'public' is the definition.
(thesaurus)
'one concealing fancy undies mostly' is the wordplay.
'one' becomes 'ace' (ace has a value of one in some card games).
'concealing' indicates putting letters inside (back).
'fancy' is an anagram indicator.
'mostly' means to remove the last letter (most but not all of the word).
'undies' with its final letter taken away is 'undie'.
'undie' is an anagram of 'udien'.
'ace' enclosing 'udien' is 'AUDIENCE'.
'in' is the link.
(Other definitions for audience that I've seen before include "Concert listeners" , "Viewers - listeners" , "Formal meeting with the Pope" , "Assembly gathered to watch, say, a play" , "The group listens or watches" .)