Old Earl receiving police discharge (7)
I believe the answer is:
explode
'discharge' is the definition.
(exploding is a kind of discharging)
'old earl receiving police' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'earl' becomes 'e'.
'receiving' is an insertion indicator.
'police' becomes 'plod' (I can't justify this - if you can you should believe this answer much more).
'ex'+'e'='exe'
'exe' enclosing 'plod' is 'EXPLODE'.
Can you help me to learn more?
(Other definitions for explode that I've seen before include "Blow up" , "Go off with a bang" , "Burst violently" , "Set off" , "Pop" .)