Advance from unsentimental banking personnel after time off (12)
I believe the answer is:
breakthrough
'advance' is the definition.
(I've seen this before)
'unsentimental banking personnel after time off' is the wordplay.
'unsentimental' becomes 'tough' (similar in meaning).
'banking' means one lot of letters goes inside another.
'personnel' becomes 'HR' (Human Resources).
'after' is a charade indicator (letters next to each other).
'time off' becomes 'break' (eg a break from work).
'tough' placed around 'hr' is 'through'.
'through' put after 'break' is 'BREAKTHROUGH'.
'from' is the link.
(Other definitions for breakthrough that I've seen before include "Sudden significant development" , "Success" , "Significant advance" , "Important development (eg in research)" , "(At last!) the solution, eg to scientific problem" .)