A material thing in a u-boat position (9)

I believe the answer is:

substance

I'm a little stuck... Click here to teach me more about this clue!

'a material thing' is the definition.
'substance' can be an answer for 'thing' (thesaurus). I am unsure of the 'a material' bit.

'u-boat position' is the wordplay.
'u-boat' becomes 'sub' (German submarine).
'position' becomes 'stance' (stance is a kind of position).
'sub'+'stance'='SUBSTANCE'

'in a' is the link.

Can you help me to learn more?

(Other definitions for substance that I've seen before include "Basic matter" , "power" , "Physical material" , "Matter or material that a thing is made of" , "wealth" .)

I've seen this clue in the Sydney Morning Herald.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.