Most unhappy person that may be seen on building site (9)

'site' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.

'most unhappy person that may be seen on' is the wordplay.
'most' means to remove the last letter.
'unhappy' becomes 'blue' (blue can mean sad or miserable).
'person' becomes 'one' ('one' can mean an unspecified person).
'that may be seen' becomes 'est' (I can't justify this - if you can you should believe this answer much more).
'on' means one lot of letters go next to another.
'blue' with its final letter taken off is 'blu'.
'one' put after 'est' is 'estone'.
'blu'+'estone'='BLUESTONE'

'building' is the link.

Can you help me to learn more?