Harmful tablet, quietly disposed of (3)
I believe the answer is:
ill
'harmful' is the definition.
(similar in meaning)
'tablet quietly disposed of' is the wordplay.
'tablet' becomes 'pill' (synonyms).
'quietly' becomes 'p' (p for piano and quiet).
'disposed of' indicates named letters should be taken away.
'pill' with 'p' taken out is 'ILL'.
(Other definitions for ill that I've seen before include "Afflicted" , "Unwell, sick" , "I will (abbrev.) (1'2)" , "Dicky" , "weak" .)