Stricken by illness, I’d allow for new treatment (4,3)
I believe the answer is:
laid low
'stricken by illness' is the definition.
(eg being laid low with the flu)
'i'd allow for new treatment' is the wordplay.
'for new treatment' indicates an anagram (letters must be processed or treated).
'id'+'allow'='idallow'
'idallow' with letters rearranged gives 'LAID LOW'.
(Other definitions for laid low that I've seen before include "Felled" , "Stricken" , "Indisposed" , "Hid away" , "Brought down" .)