Of course, these are essential (6,3,6)
I believe the answer is:
bricks and mortar
'essential' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'of course these are' is the wordplay.
I cannot quite understand how this works, but
'of' could be 'o' and 'o' is found within the answer.
'course' could be 's' and 's' is located in the answer.
'are' could be 'a' (short for 'are', historical unit of measurement) and 'a' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for bricks and mortar that I've seen before include "buildings" , "Housing" .)