Eighty are utterly in favour of second centre (9)
I believe the answer is:
fourscore
'eighty' is the definition.
(synonyms)
'utterly in favour of second centre' is the wordplay.
'utterly' indicates a 'sounds like' (homophone) clue (as uttered).
'in favour of' becomes 'for' (being for a particular cause).
'second' becomes 's' (common abbreviation).
'centre' becomes 'core' (core can mean something's centre).
'for' sounds like 'four'.
'four'+'s'+'core'='FOURSCORE'
'are' is the link.
(Other definitions for fourscore that I've seen before include "''... and seven years ago, our fathers.... (Gettysburg address)" , "Eighty" , "number" .)