Young achievers, having defect, quietly coming in second (8)
I believe the answer is:
bratpack
'young achievers' is the definition.
(group of young actors in the 1980s)
'defect quietly coming in second' is the wordplay.
'defect' becomes 'rat' (ratting is a kind of defecting).
'quietly' becomes 'p' (p for piano and quiet).
'coming in' is an insertion indicator.
'second' becomes 'back' (to second a motion is to back it).
'rat'+'p'='ratp'
'ratp' going inside 'back' is 'BRATPACK'.
'having' acts as a link.
(Another definition for bratpack that I've seen is " Group of rowdy young American film stars of he 1980s".)