Depressant (6)
'depressant' is the definition.
(I know that depressant can be written as downer)
This is the entire clue.
(Other definitions for downer that I've seen before include "Sedative" , "Depressing experience" , "Informally a depressant drug" , "In slang, a depressant drug" , "tranquilliser" .)
(Other definitions for dampener that I've seen before include "Moist kind of depressant?" , "muffler" , "Something that depresses - or moistens" , "killjoy" , "A thing that deadens" .)