One to watch succeeded young wizard (7)
I believe the answer is:
spotter
'one to watch' is the definition.
(thesaurus)
'succeeded young wizard' is the wordplay.
'succeeded' becomes 's' (genealogical abbreviation).
'young wizard' becomes 'potter' (Harry Potter).
's'+'potter'='SPOTTER'
(Other definitions for spotter that I've seen before include "Train enthusiast" , "Spy plane" , "artist such as Seurat?" , "One who looks to train" , "Observer" .)