A material thing in a u-boat position (9)
I believe the answer is:
substance
'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" .)