Insects do not start vitamin deficiency (7)
I believe the answer is:
rickets
'vitamin deficiency' is the definition.
(thesaurus)
'insects do not start' is the wordplay.
'insects' becomes 'crickets' (cricket is a type of insect).
'do not start' 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-distorting disease" , "Children's disease" , "Childhood disease caused by a deficiency of vitamin D" , "Bone disease, caused by vitamin D deficiency" .)