Slackening in mid-career assignment (9)
I believe the answer is:
remission
'slackening' is the definition.
(I've seen this before)
'mid-career assignment' is the wordplay.
'mid' says to take the centre (mid- can mean the middle of something).
'assignment' becomes 'mission' (mission is a kind of assignment).
The central letters of 'career' are 're'.
're'+'mission'='REMISSION'
'in' is the link.
(Other definitions for remission that I've seen before include "Reduction of sentence" , "Reprieve" , "Forgiveness for sins for missioner" , "improvement in patient's condition" , "Reduction of charge" .)