Time served in rebel vessel for training (9)
I believe the answer is:
cadetship
'training' is the definition.
(period spent as a cadet)
'time served in rebel vessel' is the wordplay.
'time' becomes 't' (abbreviation).
'served in' means one lot of letters goes inside another.
'rebel' becomes 'cade' (s5th-century rebel Jack Cade).
'vessel' becomes 'ship' (ship is a kind of vessel).
'cade'+'ship'='cadeship'
't' put within 'cadeship' is 'CADETSHIP'.
'for' is the link.
(Other definitions for cadetship that I've seen before include "Training position" , "In position of one being trained" , "probationary role" , "Officer training" , "Position of trainee" .)