Bullfighter miffed initially at bother, resistance (7)
I believe the answer is:
matador
'bullfighter' is the definition.
(synonyms)
'miffed initially at bother resistance' is the wordplay.
'initially' indicates taking the first letters.
'bother' becomes 'ado' (both can mean fuss or commotion).
'resistance' becomes 'r' (electronics abbreviation).
The initial letter of 'miffed' is 'm'.
'm'+'at'+'ado'+'r'='MATADOR'
(Other definitions for matador that I've seen before include "A bull-fighter.11 Beginning to develop" , "The one who kills the bull" , "who does for bulls" , "ROM data (anag)" , "Performer in the Ring" .)