Nonsense! It's almost too quiet! (4)
I believe the answer is:
tosh
'nonsense' is the definition.
(I know that tosh is a type of nonsense)
'almost too quiet' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'quiet' becomes 'sh' (both can be said to tell someone to be quiet).
'too' with its final letter taken away is 'to'.
'to'+'sh'='TOSH'
'it's' acts as a link.
(Other definitions for tosh that I've seen before include "[BLAH]" , "garbage'" , "Rubbish, nonsense" , "Pal" , "Humbug" .)