Territory league enters associations' championships (6)
I believe the answer is:
titles
'championships' is the definition.
('title' can be a synonym of 'championship')
'territory league enters associations'' is the wordplay.
'territory league' becomes 'tl' (this could be a standard abbreviation of which I'm unaware).
'enters' is an insertion indicator.
'associations'' becomes 'ties'' ('tie' can be a synonym of 'association').
'tl' put inside 'ties' is 'TITLES'.
Can you help me to learn more?
(Other definitions for titles that I've seen before include "Names or headings" , "Names for books or nobility" , "Names or honorifics" , "Names of books or lords" , "Championships" .)