Commentator’s to favour one side’s scorer (5)
I believe the answer is:
liszt
'scorer' is the definition.
(Franz Liszt)
'commentator's to favour one side's' is the wordplay.
'commentator's' indicates a 'sounds like' (homophone) clue (as it might sound from someone commentating).
'to favour one side' becomes 'list' (to list is to lean to one side).
'list' sounds like 'LISZT'.
(Other definitions for liszt that I've seen before include "Hungarian composer/pianist" , "He scored" , "Hungarian composer, d. 1886" , "Pianist, father of Cosima Wagner" , "Pianist and composer Cockneys associate with Brahms" .)