Acquisitive King invading this territory’s borders (7)
I believe the answer is:
thirsty
'acquisitive' is the definition.
Both the answer and definition are adjectives. Perhaps you can see an association between them that I can't see?
'king invading this territory's borders' is the wordplay.
'king' becomes 'R' (abbreviation of Latin rex).
'invading' means one lot of letters goes inside another.
'borders' says to hollow out the word (remove centre letters) (only the edge or border letters remain).
'territory' with its centre taken out is 'ty'.
'r' put within 'this' is 'thirs'.
'thirs'+'ty'='THIRSTY'
Can you help me to learn more?
(Other definitions for thirsty that I've seen before include "Needing moisture" , "Dry" , "Needing a tipple" , "Feeling dehydrated" , "Ardently desiring" .)