Set out to cover unknown infection (4)
I believe the answer is:
stye
'infection' is the definition.
(stye is a kind of infection)
'set out to cover unknown' is the wordplay.
'out' indicates an anagram (out can mean wrong or inaccurate).
'to cover' indicates putting letters inside.
'unknown' becomes 'y' (algebra).
'set' with letters rearranged gives 'ste'.
'ste' going around 'y' is 'STYE'.
(Other definitions for stye that I've seen before include "irritation to the viewer" , "Eyelid problem" , "nasty spot" , "makes viewer sore" , "It pains viewer" .)