Very serious complaint about a rag (6)
I believe the answer is:
tragic
'very serious' is the definition.
'complaint about a rag' is the wordplay.
'complaint' becomes 'tic' (a tic is a medical complaint).
'about' is an insertion indicator.
'tic' enclosing 'rag' is 'TRAGIC'.
(Other definitions for tragic that I've seen before include "Very sad; Schubert's 4th symphony nickname" , "Shocking" , "'Very, very sad (6)'" , "Deeply distressing" , "Disastrous, pitiful" .)