Katy did horrible thing - it causes them to drop like flies perhaps (6,9)
I believe the answer is:
insect repellent
'it causes them to drop like flies perhaps' is the definition.
'katy did horrible thing' is the wordplay.
I cannot really understand how this works, but
'horrible' could be 'repellent' and 'repellent' is found within the answer.
The remaining letters 'insect' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for insect repellent that I've seen before include "Lotion or spray against bites" , "flies away with this?" , "Citronella, say" , "It deters pest" .)