Cadet Jan possibly very close (8)
I believe the answer is:
adjacent
'very close' is the definition.
(I've seen this before)
'cadet jan possibly' is the wordplay.
'possibly' is an anagram indicator.
'cadet'+'jan'='cadetjan'
'cadetjan' anagrammed gives 'ADJACENT'.
(Other definitions for adjacent that I've seen before include "Adjoining, lying near" , "Alongside" , "Nearby" , "Next (to something)" , "Abutting" .)