Senior servant still essentially alert (6)
I believe the answer is:
butler
'senior servant' is the definition.
'butler' can be an answer for 'servant' (butler is a kind of servant). I am unsure of the 'senior' bit.
'still essentially alert' is the wordplay.
'still' becomes 'but' (both can mean nonetheless or even so).
'essentially' indicates the central letters.
The centre of 'alert' is 'ler'.
'but'+'ler'='BUTLER'
Can you help me to learn more?
(Other definitions for butler that I've seen before include "Male domestic servant" , "British novelist" , "Lofty head servant - what did he see?" , "Head servant of posh household" , "Head manservant of posh household" .)