Specialist field being security, one’s descending on N Scottish town (9)
I believe the answer is:
bailiwick
'scottish town' is the definition.
'bailiwick' can be an answer for 'town' (thesaurus). I'm not sure about the 'scottish' bit.
'specialist field being security one's descending on' is the wordplay.
'specialist field' becomes 'wick' (I can't justify this - if you can you should believe this answer much more).
'being security' becomes 'bail' (I've seen this before).
'one' becomes 'i' (Roman numeral).
'descending on' says to put letters next to each other (in a down clue, some letters go on top of others).
'bail'+'i'='baili'
'wick' put after 'baili' is 'BAILIWICK'.
'n' acts as a link.
Can you help me to learn more?
(Other definitions for bailiwick that I've seen before include "Area of interest" , "Jurisdiction of a bailiff" , "legal official's jurisdiction" , "special field" , "Jersey" .)