Infection small yet spreading (4)
I believe the answer is:
stye
'infection' is the definition.
(stye is a kind of infection)
'small yet spreading' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'spreading' is an anagram indicator.
'yet' anagrammed gives 'tye'.
's'+'tye'='STYE'
(Other definitions for stye that I've seen before include "Inflammation around the eye" , "dump" , "pain for the observer" , "Eyelid sore" , "Ocular inflammation" .)