TV transition saw three D misused (9)
I believe the answer is:
watershed
'tv transition' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'saw three d misused' is the wordplay.
'misused' is an anagram indicator.
'saw'+'three'+'d'='sawthreed'
'sawthreed' is an anagram of 'WATERSHED'.
Can you help me to learn more?
(Other definitions for watershed that I've seen before include "Event marking a turning point" , "Time after which content of TV programmes is less regulated" , "Time after which adult programmes may be shown" , "dividing line" , "Ridge between river valleys" .)