Infection starts off seeming trivial, yet escalates (4)
I believe the answer is:
Here is my best explanation:
'infection' is the definition.
(stye is a kind of infection)
'starts off seeming trivial yet escalates' is the subsidiary indication.
'starts off' says to take the initial letters.
The initial letters of 'seeming trivial yet escalates' is 'stye'.
(Other definitions for stye that I've seen before include "Eyesore?", "Eyelid swelling", "Swelling near the eye", "Eyelid inflammation", "Eye sore", "Eyelid infection", "Infection of eyelid", "Inflamed swelling on the edge of an eyelid", "Eyelid sore" and "Eye complaint".)