Small yet troublesome inflammation (4)
I believe the answer is:
stye
'inflammation' is the definition.
(I've seen this before)
'small yet troublesome' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'troublesome' indicates an anagram.
'yet' anagrammed gives 'tye'.
's'+'tye'='STYE'
(Other definitions for stye that I've seen before include "Eye problem" , "Animal pen" , "Observer's problem" , "producing eyesore" , "irritation to the viewer" .)