House dependency, conservatory, in the end leaves a mark (9)
I believe the answer is:
semicolon
'a mark' is the definition.
(semicolon is a kind of mark)
'house dependency conservatory' is the wordplay.
'house' becomes 'semi' (short for semi-detached).
'dependency conservatory' becomes 'colon' (I am not sure about this - if you are sure you should believe this answer much more).
'semi'+'colon'='SEMICOLON'
'in the end leaves' acts as a link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for semicolon that I've seen before include "Joiner" , "Item of punctuation" , "used by writers for a break" , "Punctuation mark denoting a pause in a sentence" , "half a stop" .)