Like quiet poisoner (3)
I believe the answer is:
asp
'poisoner' is the definition.
(I've seen this before)
'like quiet' is the wordplay.
'like' becomes 'as'.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'as'+'p'='ASP'
(Other definitions for asp that I've seen before include "Poisonous type" , "Joe Blake is Australian species" , "Snake of the Nile" , "Serpent" , "A glider's what's called for" .)