Sacked the chap who got out of line (5)
I believe the answer is:
'sacked the chap who' is the definition.
'fired' can be an answer for 'sacked' (both can mean dismissed from a job). I am not sure about the rest of the definition.
'got out of line' is the wordplay.
'got out of' becomes 'fi' (I can't justify this - if you can you should believe this answer much more).
'line' becomes 'red line' (red line is a kind of line).
Can you help me to learn more?
(Other definitions for fired that I've seen before include "(Of pottery) baked" , "Aroused" , "Dismissed or ignited" , "Set ablaze" , "Sacked or ignited" .)