Dismiss about fifty for being unconscientious (5*)
I believe the answer is:
'being unconscientious' is the definition.
Both the answer and definition are adjectives. Perhaps you can see an association between them that I don't see?
'dismiss about fifty' is the wordplay.
'dismiss' becomes 'sack' (synonyms).
'about' means one lot of letters goes inside another.
'fifty' becomes 'L' (Roman numeral).
'sack' placed around 'l' is 'SLACK'.
'for' is the link.
Can you help me to learn more?
(Other definitions for slack that I've seen before include "Lazy and careless" , "coal dust" , "Not taut" , "Powdery coal" , "Idle and disinclined to work" .)