Senior servant brute left in mess (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'm not certain of the 'senior' bit.
'brute left in mess' is the wordplay.
'left' becomes 'L' (common abbreviation).
'in mess' is an anagram indicator.
'brute'+'l'='brutel'
'brutel' is an anagram of 'BUTLER'.
Can you help me to learn more?
(Other definitions for butler that I've seen before include "Head servant of posh household" , "Top manservant of a posh household" , "Top servant in posh household" , "Lofty head servant - what did he see?" , "Head manservant of posh household" .)