Well, what to do with crumbling mortar is a painful subject (4,5)
I believe the answer is:
sore point
'well what to do with crumbling mortar' is the definition.
I don't know anything about this answer so I can't judge whether this works.
'painful subject' is the wordplay.
'painful' becomes 'sore' (synonyms).
'subject' becomes 'point' (I can't justify this - if you can you should give a lot more credence to this answer).
'sore'+'point'='SORE POINT'
'is a' acts as a link.
Can you help me to learn more?
(Other definitions for sore point that I've seen before include "Annoying issue" , "Touchy matter" , "grievance" .)