Could be another's fishing ground (5,3)
I believe the answer is:
north sea
'fishing ground' is the definition.
The definition and answer can be both natural objects as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'could be another's' is the wordplay.
'could be' indicates an anagram.
'anothers' is an anagram of 'NORTH SEA'.
Can you help me to learn more?
(Other definitions for north sea that I've seen before include "Part of the Atlantic to the east of Great Britain" , "British oil/gas area" , "It's not land to our East" , "Waters east of the UK" , "Oil producer" .)