All done at last, exactly what's needed (10)
I believe the answer is:
everything
'all' is the definition.
(I've seen this before)
'done at last exactly what's needed' is the wordplay.
'at last' indicates one should take the final letters.
'exactly what's needed' becomes 'verything' ('the very thing' can mean exactly what's needed).
The last letter of 'done' is 'e'.
'e'+'verything'='EVERYTHING'
(Other definitions for everything that I've seen before include "great deal" , "The whole lot" , "the works" , "The whole shebang" , "All items" .)