Send one cat right away (2,4)
I believe the answer is:
at once
'right away' is the definition.
(I've seen this before)
'send one cat' is the wordplay.
'send' indicates an anagram (I've seen 'sends' mean this (send can mean excite or stir)).
'one'+'cat'='onecat'
'onecat' anagrammed gives 'AT ONCE'.
(Other definitions for at once that I've seen before include "This instant - no delay (2,4)" , "right away" , "This minute!" , "on the spot" , "without pause" .)