Swelling small yet explosive (4)
I believe the answer is:
stye
'swelling' is the definition.
(stye is a kind of swelling)
'small yet explosive' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'explosive' indicates an anagram.
'yet' anagrammed gives 'tye'.
's'+'tye'='STYE'
(Other definitions for stye that I've seen before include "Eyelid affliction" , "Eyelid infection" , "Swelling near the eye" , "It pains viewer" , "Infection on the edge of the eye" .)