Disease sees lamb mostly destroyed (7)
I believe the answer is:
measles
'disease' is the definition.
(measles is a kind of disease)
'sees lamb mostly destroyed' is the wordplay.
'mostly' means to remove the last letter.
'destroyed' indicates anagramming the letters.
'sees'+'lamb'='seeslamb'
'seeslamb' with its last letter taken off is 'seeslam'.
'seeslam' anagrammed gives 'MEASLES'.
(Other definitions for measles that I've seen before include "Childhood malady" , "An infectious fever" , "Ailment" , "Infectious disease covering body with red spots" , "Rubeola" .)