As many illnesses are, primarily requiring drug in form of a tablet (9)
I believe the answer is:
treatable
'as many illnesses are' is the definition.
The answer and definition are different parts of speech. However, past participle verbs and adjectives occasionally define each other.
'primarily requiring drug in form of a tablet' is the wordplay.
'primarily' indicates taking the first letters.
'drug' becomes 'e' (E is another name for ecstasy).
'in' is an insertion indicator.
'form of a tablet' becomes 'tatable' (I can't justify this - if you can you should believe this answer much more).
The first letter of 'requiring' is 'r'.
'r'+'e'='re'
're' placed inside 'tatable' is 'TREATABLE'.
(Other definitions for treatable that I've seen before include "one can deal with" , "Can be handled" .)