Spoil one's appearance: quietly going inside (6)
I believe the answer is:
impair
'spoil' is the definition.
(I know that spoil can be written as impair)
'one's appearance quietly going inside' is the wordplay.
'one's' becomes 'I'm' ('one is' is a formal way of saying 'I am').
'appearance' becomes 'air' (both can mean an impression or character).
'quietly' becomes 'p' (musical symbol).
'going inside' means one lot of letters goes inside another.
'im'+'air'='imair'
'imair' placed around 'p' is 'IMPAIR'.
(Other definitions for impair that I've seen before include "Harm" , "blunt" , "'Mar, spoil (6)'" , "Spoil strength, quality of" , "Hobble" .)