Service winner overcomes resistance in overwhelming victory (8)
I believe the answer is:
massacre
'overwhelming victory' is the definition.
'service winner overcomes resistance' is the wordplay.
'service' becomes 'mass' (I've seen this in other clues**).
'winner' becomes 'ace'.
'overcomes' means one lot of letters goes inside another.
'resistance' becomes 'r' (abbreviation).
'ace' enclosing 'r' is 'acre'.
'mass'+'acre'='MASSACRE'
'in' acts as a link.
(Other definitions for massacre that I've seen before include "Brutal slaughter" , "Widespread killing" , "Indiscriminate killing of large numbers" , "The slaughter of many" , "Slaughter of large number of people" .)