Inflammation in son yet to be treated (4)
I believe the answer is:
stye
'inflammation' is the definition.
(I've seen this before)
'son yet to be treated' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'to be treated' indicates an anagram.
'yet' with letters rearranged gives 'tye'.
's'+'tye'='STYE'
'in' acts as a link.
(Other definitions for stye that I've seen before include "dump" , "pain for the observer" , "Inflamed swelling on the edge of an eyelid" , "It pains viewer" , "Eyelid infection" .)