Law group's heading South the writer's contracted for case (10)
I believe the answer is:
possessive
'for case' is the definition.
(possessive case is a kind of case)
'law group's heading south the writer's contracted' is the wordplay.
'law group' becomes 'posse' (I've seen this in another clue).
'heading south' becomes 's' (abbreviation).
'the writer's contracted' becomes 'sive' (I can't explain this - if you can you should believe this answer much more).
'posse'+'s'+'sive'='POSSESSIVE'
Can you help me to learn more?
(Other definitions for possessive that I've seen before include "Desiring to retain what one owns" , "jealousy" , "Jealous; controlling" , "Case" , "Unwilling to share" .)