Loose end of coal in fire (5)
I believe the answer is:
slack
'loose' is the definition.
(I know that loose can be written as slack)
'end of coal in fire' is the wordplay.
'end of' indicates one should take the final letters.
'in' is an insertion indicator.
'fire' becomes 'sack' (dismiss someone from their job).
The final letter of 'coal' is 'l'.
'l' placed inside 'sack' is 'SLACK'.
(Other definitions for slack that I've seen before include "skive" , "Not taut" , "Limp - negligent" , "Powdery coal" , "Loose" .)