Not put up in one spot without delay (2,4)
I believe the answer is:
at once
'without delay' is the definition.
(I've seen this in another clue)
'not put up in one' is the wordplay.
'put up' says the letters should be written in reverse.
'in' indicates putting letters inside.
'one' becomes 'ace' (ace has a value of one in some card games).
'not' written backwards gives 'ton'.
'ton' going within 'ace' is 'AT ONCE'.
'spot' is the link.
This may not be correct. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for at once that I've seen before include "A no etc (anag)" , "Immediately, this instant" , "Immediately, without delay" , "without pause" , "right away" .)