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