Nobleman welcomes resistance to James the First from old city - he's a liar and all The Court knows it (8)
I believe the answer is:
perjurer
'old city he's a liar and all the court knows it' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'nobleman welcomes resistance to james the first' is the wordplay.
'nobleman' becomes 'peer' (peer is a kind of nobleman).
'welcomes' is an insertion indicator (in sense of 'invites in').
'resistance' becomes 'r' (abbreviation).
'to james the first' becomes 'jur' (I can't explain this - if you can you should believe this answer much more).
'r'+'jur'='rjur'
'peer' enclosing 'rjur' is 'PERJURER'.
'from' acts as a link.
Can you help me to learn more?
(Other definitions for perjurer that I've seen before include "One lying on oath" , "Person who lies under oath" , "False witness" .)