Dismissal involves 50 becoming careless (5)
I believe the answer is:
slack
'careless' is the definition.
('slack' can be a synonym of 'careless')
'dismissal involves 50' is the wordplay.
'dismissal' becomes 'sack' ('sack' can be a synonym of 'dismissal').
'involves' means one lot of letters goes inside another.
'50' becomes 'l' (roman numeral).
'sack' enclosing 'l' is 'SLACK'.
'becoming' is the link.
(Other definitions for slack that I've seen before include "Limp - negligent" , "bits of coal" , "Loose" , "Lazy and careless" , "Lax" .)