Order one cat now (2,4)
I believe the answer is:
at once
'now' is the definition.
(I've seen this before)
'order one cat' is the wordplay.
'order' indicates anagramming the letters.
'one'+'cat'='onecat'
'onecat' with letters rearranged gives 'AT ONCE'.
(Other definitions for at once that I've seen before include "This instant, hop to it" , "without pause" , "right away" , "This minute!" , "A no etc (anag)" .)