Another illness overwhelms one in sad isolation (10)
I believe the answer is:
loneliness
'sad isolation' is the definition.
'loneliness' can be an answer for 'isolation' (loneliness is a kind of isolation). I am not certain of the 'sad' bit.
'another illness overwhelms one' is the wordplay.
'another' indicates an anagram.
'overwhelms' means one lot of letters goes inside another.
'illness' with letters rearranged gives 'lliness'.
'lliness' going around 'one' is 'LONELINESS'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for loneliness that I've seen before include "Desolation" , "Unwelcome solitude" , "Solitary feeling" , "Feeling of isolation" .)