Cadet Jan moved alongside (8)
I believe the answer is:
adjacent
'alongside' is the definition.
(I've seen this before)
'cadet jan moved' is the wordplay.
'moved' indicates anagramming the letters.
'cadet'+'jan'='cadetjan'
'cadetjan' with letters rearranged gives 'ADJACENT'.
(Other definitions for adjacent that I've seen before include "Nearby" , "next door" , "cheek by jowl" , "Contiguous" , "Neither opposite nor hypotenuse?" .)