Gruesome as lesion's concealing disease (7)
I believe the answer is:
measles
'disease' is the definition.
(measles is a kind of health problem)
'gruesome as lesion's concealing' is the wordplay.
'concealing' indicates a hidden word.
'MEASLES' is hidden in the letters of 'gruesome as lesions'.
(Other definitions for measles that I've seen before include "Ailment" , "An infectious fever" , "Morbilli is technical term for this illness" , "Childhood malady" , "Viral disease with rash" .)