Lunettes designed to upset (8)
I believe the answer is:
unsettle
'upset' is the definition.
(unsettling is a kind of upseting)
'lunettes designed' is the wordplay.
'designed' indicates anagramming the letters (designed with the letters).
'lunettes' anagrammed gives 'UNSETTLE'.
'to' is the link.
(Other definitions for unsettle that I've seen before include "Trouble" , "Upset" , "'Disturb, make uneasy (8)'" , "Faze" , "Disturb, make anxious" .)