King imprisoned in keep gets to prepare for a new job (7)
I believe the answer is:
retrain
'to prepare for a new job' is the definition.
(train for a new career)
'king imprisoned in keep' is the wordplay.
'king' becomes 'R' (abbreviation for rex, king in Latin).
'imprisoned in' means one lot of letters goes inside another.
'keep' becomes 'retain' (retaining is a kind of keeping).
'r' going inside 'retain' is 'RETRAIN'.
'gets' is the link.
(Other definitions for retrain that I've seen before include "Prepare for new job" , "Acquire new skills" , "Take a different course" , "Teach fresh skills" , "Be taught new skills" .)