Sad jerk admits scrap (6)
I believe the answer is:
tragic
'sad' is the definition.
(I've seen this before)
'jerk admits scrap' is the wordplay.
'jerk' becomes 'tic' (I've seen this before).
'admits' is an insertion indicator.
'scrap' becomes 'rag' (I've seen this before).
'tic' enclosing 'rag' is 'TRAGIC'.
(Other definitions for tragic that I've seen before include "Extremely sad; Mahler's 6th nickname" , "'Very, very sad (6)'" , "Extremely sad and distressing" , "Grievous" , "Greatly distressing" .)