There's profundity in what a plumber may find (5)
I believe the answer is:
depth
'find' is the definition.
The definition and answer can be both related to thinking as well as being singular nouns.
Perhaps there's a link between them I don't understand?
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for depth that I've seen before include "Profoundness" , "sometimes leading to charge for sailors at war?" , "Measurement from front to back" , "Distance from top down as in water" , "Military life; vulgarly artificial" .)