He postulates about the spout when they just need to get on with laying what's left. (5)
I believe the answer is:
slate
'he postulates about the spout when they just need to get on with laying what's left' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'he postulates about the' is the wordplay.
'he postulates' becomes 'slae' (I can't explain this - if you can you should believe this answer much more).
'about' indicates putting letters inside.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'slae' going around 't' is 'SLATE'.
Can you help me to learn more?
(Other definitions for slate that I've seen before include "Scold; hard rock" , "Roofing tile" , "Blast" , "A roof-tile; criticise heavily" , "Criticise - outdated school equipment" .)