Loud, I tease to upset - and to keep quiet (6)
I believe the answer is:
garish
'loud' is the definition.
('garish' can be a synonym of 'loud')
'i tease to upset and to keep quiet' is the wordplay.
'tease' becomes 'rag' ('rag' can be a synonym of 'tease'**).
'to upset' is a reversal indicator.
'and' means one lot of letters go next to another.
'to keep quiet' becomes 'sh'.
'i'+'rag'='irag'
'irag' backwards is 'gari'.
'gari'+'sh'='GARISH'
(Other definitions for garish that I've seen before include "Obtrusively bright" , "Loud" , "Colourful in a vulgar manner" , "Showy, lurid" , "Hag, sir, could be gaudy" .)