Bitterness with operation, move fast (6)
I believe the answer is:
gallop
'move fast' is the definition.
(I've seen this before)
'bitterness with operation' is the wordplay.
'bitterness' becomes 'gall' ('gall' can be a synonym of 'bitterness').
'with' is a charade indicator (letters next to each other).
'operation' becomes 'op'.
'gall'+'op'='GALLOP'
(Other definitions for gallop that I've seen before include "Animal's fast pace" , "'At GPO, all can go at this pace (6)'" , "Hurry (on horseback?)" , "Horse's fastest pace" , "Horse's run" .)