Disease affected legs, lower part first (8)
I believe the answer is:
shingles
'disease' is the definition.
(shingles is a kind of disease)
'affected legs lower part first' is the wordplay.
'affected' is an anagram indicator.
'lower part' becomes 'shin' (a lower part of the body).
'first' says to put letters next to each other.
'legs' anagrammed gives 'gles'.
'gles' after 'shin' is 'SHINGLES'.
(Other definitions for shingles that I've seen before include "Viral inflamation, with rash of painful blisters" , "Painful virus disease or roof tiles" , "Kind of roof-tiles, or painful disease with rash" , "Wooden tiles and painful condition" , "Roof tiles or painful ailment" .)