“Disease,” with the seventh letter missing (4)
I believe the answer is:
gout
'disease' is the definition.
(I know that gout is a type of arthritis)
'with the seventh letter missing' is the wordplay.
'with' means one lot of letters go next to another.
'the seventh letter' becomes 'g' (in the alphabet).
'missing' becomes 'out' (I've seen this in another clue).
'g' next to 'out' is 'GOUT'.
(Other definitions for gout that I've seen before include "Toe-inflammation disease" , "Swelling disease" , "Painful inflammation of the foot" , "Complaint" , "Inflammatory disease" .)