Sound of awful insects carrying source of terrible disease (8)
I believe the answer is:
diabetes
'disease' is the definition.
(diabetes is a kind of gland disease)
'sound of awful insects carrying source of terrible' is the wordplay.
'sound of awful' becomes 'dia' (I can't explain this - if you can you should believe this answer much more).
'insects' becomes 'bees' (bee is a kind of insect).
'carrying' means one lot of letters goes inside another.
'source of' suggests taking the first letters.
The first letter of 'terrible' is 't'.
'bees' going around 't' is 'betes'.
'dia'+'betes'='DIABETES'
(Other definitions for diabetes that I've seen before include "Effect of lack of insulin" , "Disorder linked to insulin" , "Disease -- a side bet (anag)" , "illness" , "medical condition" .)