Stiff notice stopping one in the underground? (7)
'stiff' is the definition.
'notice stopping one in the underground?' is the wordplay.
'notice' becomes 'ad' (both can mean an advertisement).
'stopping' means one lot of letters goes inside another (stopping can mean blocking a gap).
'one in the underground?' becomes 'caver' (I can't justify this - if you can you should believe this answer much more).
'ad' going inside 'caver' is 'CADAVER'.
Can you help me to learn more?
(Other definitions for cadaver that I've seen before include "Human body dissected by medical students", "Remains of a human", "Stiff", "Human corpse", "Dead body, corpse".)