Appear laid up nursing a case of social disease (7)
I believe the answer is:
measles
'disease' is the definition.
(measles is a kind of disease)
'appear laid up nursing a case of social' is the wordplay.
'appear' becomes 'seem' ('seem' can be a synonym of 'appear').
'laid up' says the letters should be written in reverse.
'nursing' means one lot of letters goes inside another.
'case of' suggests removing the centre (outsides of).
'social' with its middle taken out is 'sl'.
'seem' written backwards gives 'mees'.
'a'+'sl'='asl'
'mees' going around 'asl' is 'MEASLES'.
(Other definitions for measles that I've seen before include "Contagious disease in children" , "Childhood viral disease" , "Infectious disease covering body with red spots" , "Viral disease with rash" , "An infectious fever" .)