Put old cement round last year's droppings (9)

'last year's droppings' is the definition.
The answer and definition can be both substances as well as being singular nouns.
Perhaps you can see a link between them that I don't see?

'put old cement round' is the wordplay.
'put' becomes 'r' (I can't explain this - if you can you should give a lot more credence to this answer).
'old' becomes 'ex' (prefix meaning former or onetime).
'round' indicates putting letters inside.
'ex'+'cement'='excement'
'r' put inside 'excement' is 'EXCREMENT'.

Can you help me to learn more?