Break from assignment by junior doctor finishing early (12)
I believe the answer is:
intermission
'break' is the definition.
(I've seen this before)
'assignment by junior doctor finishing early' is the wordplay.
'assignment' becomes 'mission' (mission is a kind of assignment).
'by' means one lot of letters go next to another.
'junior doctor' becomes 'intern' (I've seen this before).
'finishing early' means to remove the last letter.
'intern' with its last letter removed is 'inter'.
'mission' put after 'inter' is 'INTERMISSION'.
'from' is the link.
(Other definitions for intermission that I've seen before include "Theatre's half-time" , "Entr'acte" , "Pause, break" , "Interval" , "Break; respite" .)