Stones you'd be mad to walk on! (5,6)
I believe the answer is:
crazy paving
'stones you'd' is the definition.
The definition suggests a singular noun which matches the answer.
'mad to walk on' is the wordplay.
'mad' becomes 'crazy' (synonyms).
'to walk on' becomes 'paving' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'crazy'+'paving'='CRAZY PAVING'
'be' acts as a link.
Can you help me to learn more?
(Other definitions for crazy paving that I've seen before include "Garden path style" , "Irregular style for, eg, garden paths" , "Mad slabs?" , "Pieces of irregular stone slabs fitted together" , "Irregular-shaped slabs" .)