Insects scratching head, leading to disease (7)
I believe the answer is:
rickets
'disease' is the definition.
(rickets is a kind of health problem)
'insects scratching head' is the wordplay.
'insects' becomes 'crickets' (cricket is a kind of insect).
'scratching head' means to remove the first letter (the word\'s head is removed).
'crickets' with its initial letter taken away is 'RICKETS'.
'leading to' acts as a link.
(Other definitions for rickets that I've seen before include "Children's disease" , "Childhood disease caused by a deficiency of vitamin D" , "Bone-distorting disease" , "Bone disease, caused by vitamin D deficiency" .)