Small yet volatile infection (4)
I believe the answer is:
stye
'infection' is the definition.
(stye is a kind of infection)
'small yet volatile' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size**).
'volatile' indicates anagramming the letters.
'yet' with letters rearranged gives 'tye'.
's'+'tye'='STYE'
(Other definitions for stye that I've seen before include "Inflammation on eyelid" , "Eyelid problem" , "Eyelid affliction" , "Inflamed swelling on the edge of an eyelid" , "Ocular affliction" .)