Casual agreement to hold career back leads to destitution (6)
I believe the answer is:
penury
'destitution' is the definition.
(I've seen this before)
'casual agreement to hold career back' is the wordplay.
'casual agreement' becomes 'yep' (informal version of 'yes').
'to hold' is an insertion indicator.
'career' becomes 'run' (I've seen this before).
'back' says the letters should be written backwards.
'yep' going around 'run' is 'yrunep'.
'yrunep' written backwards gives 'PENURY'.
'leads to' acts as a link.
(Other definitions for penury that I've seen before include "Indigence, destitution" , "State of extreme poverty" , "State of poverty or destitution" , "Extreme poverty or indigence" , "Real poverty" .)