Saint has the old eye problem (4)
I believe the answer is:
stye
'eye problem' is the definition.
(I've seen this before)
'saint has the old' is the wordplay.
'saint' becomes 'st' (abbreviation).
'has' means one lot of letters go next to another.
'the old' becomes 'ye' (resembles a historical spelling of 'the').
'st'+'ye'='STYE'
(Other definitions for stye that I've seen before include "Eye blemish" , "Viewer's complaint" , "It pains viewer" , "Inflamed swelling on the edge of an eyelid" , "producing eyesore" .)