A hack or policeman getting out of breath (4)
I believe the answer is:
chop
'a hack' is the definition.
(I know that hack can be written as chop)
'policeman getting out of breath' is the wordplay.
'policeman' becomes 'cop' (slang term).
'getting out of' means one lot of letters goes inside another (to be outside something can mean to be around it).
'breath' becomes 'h' (I can't justify this - if you can you should believe this answer much more).
'cop' enclosing 'h' is 'CHOP'.
'or' acts as a link.
Can you help me to learn more?
(Other definitions for chop that I've seen before include "Cut - a piece of meat" , "Cut into small pieces" , "Piece of meat - the sack" , "Thick slice of meat including rib" , "Cut of meat - the sack" .)