Musician alternatively upsetting giants (8)
I believe the answer is:
organist
'musician' is the definition.
(I know that organist is a type of musician)
'alternatively upsetting giants' is the wordplay.
'alternatively' becomes 'or'.
'upsetting' indicates an anagram.
'giants' is an anagram of 'ganist'.
'or'+'ganist'='ORGANIST'
(Other definitions for organist that I've seen before include "One plays in church usually" , "Cathedral musician" , "Accompanist, keyboard player" , "Church instr. player" , "Roasting (anag)" .)