Disease insects head off (7)
I believe the answer is:
rickets
'disease' is the definition.
(rickets is a kind of disease)
'insects head off' is the wordplay.
'insects' becomes 'crickets' (cricket is a kind of insect).
'head off' means to remove the first letter.
'crickets' with its first letter taken away is 'RICKETS'.
(Other definitions for rickets that I've seen before include "Bone disease, caused by vitamin D deficiency" , "Children's disease" , "Childhood disease caused by a deficiency of vitamin D" , "Bone-distorting disease" .)