Not put back in one spot straightaway (2,4)
I believe the answer is:
at once
'spot straightaway' is the definition.
'at once' can be an answer for 'straightaway' (both can mean immediately). I am not sure about the 'spot' bit.
'not put back in one' is the wordplay.
'put back' is a reversal indicator.
'in' indicates putting letters inside.
'one' becomes 'ace' (ace has a value of one in some card games).
'not' back-to-front is 'ton'.
'ton' placed within 'ace' is 'AT ONCE'.
Can you help me to learn more?
(Other definitions for at once that I've seen before include "Octane (anag)" , "This instant, without delay" , "This instant - no delay (2,4)" , "This minute!" , "without pause" .)