Manservant nevertheless left Queen (6)
I believe the answer is:
'manservant' is the definition.
(butler is a kind of manservant)
'nevertheless left queen' is the wordplay.
'nevertheless' becomes 'but' (I've seen this before).
'left' becomes 'l' (common abbreviation).
'queen' becomes 'er' (abbreviation for Elizabeth Regina).
(Other definitions for butler that I've seen before include "Lofty head servant - what did he see?" , "Top manservant of a posh household" , "Chief male household servant" , "in charge of port?" , "Senior servant of a household" .)