... assurance to win tight set (7)
I believe the answer is:
earnest
'assurance' is the definition.
The answer and definition can be both related to property as well as being singular nouns.
Maybe you can see a link between them that I don't see?
'win tight set' is the wordplay.
'win' becomes 'earn' (I've seen this before).
'tight' indicates anagramming the letters (tight can mean drunk).
'set' is an anagram of 'est'.
'earn'+'est'='EARNEST'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for earnest that I've seen before include "Sincere in intention" , "Sincere, solemn" , "Sincere; determined" , "Eager, sincere" , "Fervent - down payment" .)