Saint with the old eye swelling (4)
I believe the answer is:
stye
'eye swelling' is the definition.
'stye' can be an answer for 'swelling' (stye is a kind of swelling). I am unsure of the 'eye' bit.
'saint with the old' is the wordplay.
'saint' becomes 'st'.
'with' is a charade indicator (letters next to each other).
'the old' becomes 'ye' (resembles a historical spelling of 'the').
'st'+'ye'='STYE'
Can you help me to learn more?
(Other definitions for stye that I've seen before include "Seeing problem" , "Infected swelling of the eye" , "Eyelid affliction" , "Eyelid sore" , "Eyelid problem" .)