Old king has what's needed erected all at once (5)
I believe the answer is:
tutti
'all at once' is the definition.
(musical instruction)
'old king has what's needed erected' is the wordplay.
'old king' becomes 'tut' (King Tut).
'has' says to put letters next to each other.
'what's needed' becomes 'it' (I am not sure about this - if you are sure you should believe this answer much more).
'erected' says the letters should be written in reverse (in a down clue, letters go up).
'it' written backwards gives 'ti'.
'tut'+'ti'='TUTTI'
Can you help me to learn more?
(Other definitions for tutti that I've seen before include "musical direction" , "With all instruments playing at once (music)" , "Composer of Carmina Burana" , "Orchestral passage" , "Passage using all instruments" .)