A new hotline must be organised at once (2,3,4)
I believe the answer is:
on the nail
'at once' is the definition.
(I've seen this in another clue)
'a new hotline must be organised' is the wordplay.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'must be organised' indicates an anagram.
'a'+'n'+'hotline'='anhotline'
'anhotline' anagrammed gives 'ON THE NAIL'.
(Other definitions for on the nail that I've seen before include "immediately" , "Where a coat may be hung" , "Promptly" , "Exactly" , "Where varnish is put" .)