Party that's only half discovered (5)
I believe the answer is:
disco
'party' is the definition.
(dance party)
'half discovered' is the wordplay.
'half' indicates taking half.
'discovered' with half the letters taken is 'DISCO'.
'that's only' is the link.
(Other definitions for disco that I've seen before include "Modern dance hall or event, in brief" , "Modern dancehall of youth" , "Type of music" , "Nightclub with dancing to recorded music" , "'Genre of dance music, popular in the 1970s (5)'" .)