Pair of hacks get a move on! (4,4)
I believe the answer is:
chop chop
'get a move on' is the definition.
(phrase used to tell someone to hurry up)
'pair of hacks' is the wordplay.
'pair of' says to double the letters (I've seen 'pair' mean this).
'hacks' becomes 'chop' (I can't explain this - if you can you should believe this answer much more).
'chop' doubled is 'CHOP-CHOP'.
(Other definitions for chop chop that I've seen before include "repeated punches" , "Promptly!" , "Hurry up!" , "Fast" , "with expedition" .)