Inn with salty nuts -- tons swallowed at once (9)
I believe the answer is:
instantly
'at once' is the definition.
(I've seen this before)
'inn with salty nuts tons swallowed' is the wordplay.
'with' means one lot of letters go next to another.
'nuts' is an anagram indicator (nuts can mean crazy).
'tons' becomes 't'.
'swallowed' means one lot of letters goes inside another.
'inn' put after 'salty' is 'saltyinn'.
'saltyinn' with letters rearranged gives 'insantly'.
'insantly' placed around 't' is 'INSTANTLY'.
(Other definitions for instantly that I've seen before include "Immediately, forthwith" , "Without delay or hesitation" , "With no delay, this very second" .)