Agitated Britten finally added a solo aria with no life at all (4,2,1,8)
I believe the answer is:
dead as a doornail
'with no life at all' is the definition.
'agitated britten finally added a solo aria' is the wordplay.
'agitated' indicates anagramming the letters.
'finally' says to take the final letters.
The final letter of 'britten' is 'n'.
'n'+'added'+'a'+'solo'+'aria'='naddedasoloaria'
'naddedasoloaria' with letters rearranged gives 'DEAD AS A DOORNAIL'.
(Other definitions for dead as a doornail that I've seen before include "Like one driven through entrance" , "just like a dodo!" , "far from quick!" .)