Service expert welcoming Queen's total defeat (8)
I believe the answer is:
massacre
'total defeat' is the definition.
'service expert welcoming queen's' is the wordplay.
'service' becomes 'mass' (I've seen this in other clues).
'expert' becomes 'ace' (ace can informally mean very good).
'welcoming' is an insertion indicator (in sense of 'inviting in').
'queen' becomes 'r' (r is abbreviation regina. e.g. in ER ).
'ace' placed around 'r' is 'acre'.
'mass'+'acre'='MASSACRE'
(Other definitions for massacre that I've seen before include "Kill" , "Inflict a heavy defeat" , "Slaughter of large number of people" , "Carnage" , "butcher" .)