Bacterial infection seemingly troubling your eye, firstly (4)
I believe the answer is:
stye
'bacterial infection' is the definition.
'stye' can be an answer for 'infection' (stye is a kind of infection). I am not certain of the 'bacterial' bit.
'seemingly troubling your eye firstly' is the wordplay.
'firstly' indicates taking the first letters.
The initial letters of 'seemingly troubling your eye' is 'STYE'.
Can you help me to learn more?
(Other definitions for stye that I've seen before include "Seeing problem" , "irritation to the viewer" , "Eyelid sore" , "Eyelid infection" , "Infected swelling of the eye" .)