Observer of second boy wizard (7)
I believe the answer is:
spotter
'observer' is the definition.
(I know that spotter is a type of observer)
'second boy wizard' is the wordplay.
'second' becomes 's' (common abbreviation).
'boy wizard' becomes 'potter' (Harry Potter).
's'+'potter'='SPOTTER'
'of' acts as a link.
(Other definitions for spotter that I've seen before include "Spy plane" , "One who looks to train" , "scout?" , "Observer" , "artist such as Seurat?" .)