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 can't explain this - if you can you should give a lot more credence to this answer).
'semi'+'colon'='SEMICOLON'
'in the end leaves' acts as a link.
I am not very happy about this link. 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" .)