Lion following, being after one’s territory (4,2,3)
I believe the answer is:
isle of man
'territory' is the definition.
(dependent territory of the UK)
'lion following being after one's' is the wordplay.
'lion' becomes 'leo' ('lion' in Latin).
'following' becomes 'f' (used when citing page numbers etc).
'being' becomes 'man' (human being).
'after' means one lot of letters go next to another.
'one's' becomes 'I's' (Roman numeral).
'leo'+'f'+'man'='leofman'
'leofman' put after 'is' is 'ISLE OF MAN'.
(Other definitions for isle of man that I've seen before include "Where to find the House of Keys" , "Its capital is Douglas and its cats are tailless" , "Here are Peel, Douglas" , "British outpost of the Gaels" , "'It's in the Irish Sea (4,2,3)'" .)