Tablet initially prescribed wrongly (4)
I believe the answer is:
pill
'tablet' is the definition.
(I know that tablet can be written as pill)
'initially prescribed wrongly' is the wordplay.
'initially' says to take the initial letters.
'wrongly' becomes 'ill' (as in ill-advised, ill-gotten etc).
The first letter of 'prescribed' is 'p'.
'p'+'ill'='PILL'
(Other definitions for pill that I've seen before include "Medication" , "may be the cure" , "Dreadful person" , "Medicinal tablet" , "Medicinal pellet" .)