What’s this?; a house, almost a settlement (9)
I believe the answer is:
semicolon
'what's this?' is the definition.
The definition suggests a singular noun which matches the answer.
'a house almost a settlement' is the wordplay.
'a house' becomes 'semi' (I've seen this before).
'almost' means to remove the last letter (most of the word but not all of it).
'a settlement' becomes 'colony' ('colony' can be a synonym of 'settlement').
'colony' with its last letter removed is 'colon'.
'semi'+'colon'='SEMICOLON'
(Other definitions for semicolon that I've seen before include "Item of punctuation" , "used by writers for a break" , "can cut sentence" , "Punctuation mark denoting a pause in a sentence" , "Joiner" .)