Afflicted, become ill without treatment recommended initially (8)
I believe the answer is:
stricken
'afflicted' is the definition.
('stricken' can be a synonym of 'afflicted')
'become ill without treatment recommended initially' is the wordplay.
'become ill' becomes 'sicken' (I've seen this before).
'without' indicates putting letters inside ('without' can be similar in meaning to 'outside').
'initially' indicates taking the first letters.
The first letters of 'treatment recommended' is 'tr'.
'sicken' enclosing 'tr' is 'STRICKEN'.
(Other definitions for stricken that I've seen before include "Affected by illness or grief - like a blow?" , "Seriously afflicted" , "Word that can be appended to both 1 and 2 down" , "Seriously affected (by, eg, condition)" , "Badly affected" .)