Patchy skin condition spreading into leg (7)
I believe the answer is:
lentigo
'patchy skin condition' is the definition.
(condition causing patches on skin)
'spreading into leg' is the wordplay.
'spreading' indicates anagramming the letters.
'into'+'leg'='intoleg'
'intoleg' is an anagram of 'LENTIGO'.
(Other definitions for lentigo that I've seen before include "Spots" , "skin discoloration" , "Skin condition" , "freckles" , "Skin disease" .)