In favour of career measure bringing rapid development (13)
I believe the answer is:
proliferation
'rapid development' is the definition.
(proliferation means rapid spread or expansion)
'in favour of career measure' is the wordplay.
'in favour of' becomes 'pro' (being pro a particular cause).
'career' becomes 'life' (career can mean someone's life or past).
'measure' becomes 'ration' (a measured amount of something).
'pro'+'life'+'ration'='PROLIFERATION'
'bringing' acts as a link.
(Other definitions for proliferation that I've seen before include "Increase (of nuclear weapons)" , "Increase in numbers" .)