Fire about fifty for being lax (5)
I believe the answer is:
slack
'being lax' is the definition.
(similar in meaning)
'fire about fifty' is the wordplay.
'fire' becomes 'sack' (dismiss someone from their job).
'about' means one lot of letters goes inside another.
'fifty' becomes 'l' (Roman numeral).
'sack' going around 'l' is 'SLACK'.
'for' is the link.
(Other definitions for slack that I've seen before include "Idle and disinclined to work" , "Lazy and careless" , "bits of coal" , "Powdery coal" , "Flaccid" .)