Dead old stage performer lay exhausted (7)
I believe the answer is:
exactly
'dead' is the definition.
(I've seen this before)
'old stage performer lay exhausted' is the wordplay.
'old stage performer' becomes 'exact' (I can't justify this - if you can you should give a lot more credence to this answer).
'exhausted' means to remove the middle letters (the word is emptied out or exhausted).
'lay' with its middle taken out is 'ly'.
'exact'+'ly'='EXACTLY'
Can you help me to learn more?
(Other definitions for exactly that I've seen before include "On the button" , "Precisely thus" , "on the spot" , "In precise terms" , "how one might express agreement?" .)