New test includes an upper-class disease (7)
I believe the answer is:
tetanus
'disease' is the definition.
(tetanus is a kind of health problem)
'new test includes an upper-class' is the wordplay.
'new' indicates anagramming the letters.
'includes' means one lot of letters goes inside another.
'upper-class' becomes 'u' (similar in meaning).
'test' with letters rearranged gives 'tets'.
'an'+'u'='anu'
'tets' placed around 'anu' is 'TETANUS'.
(Other definitions for tetanus that I've seen before include "Infection from wound" , "Locker" , "Attunes (anag) - muscular illness" , "Disease causing muscular spasms" , "Disease causing muscle spasms" .)