Private about to enter camp (6)
I believe the answer is:
secret
'private' is the definition.
(I know that private can be written as secret)
'about to enter camp' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'to enter' means one lot of letters goes inside another.
'camp' becomes 'sect' (sect is a kind of camp).
're' inserted inside 'sect' is 'SECRET'.
(Other definitions for secret that I've seen before include "Hidden; unknown" , "Not seen or know by others" , "Private; classified" , "Not known by others" , "Confidential or mysterious" .)