Absolutely, supporter follows right bearing (6)
I believe the answer is:
really
'absolutely' is the definition.
(thesaurus)
'supporter follows right bearing' is the wordplay.
'supporter' becomes 'ally' (ally can mean a supporter or friend).
'follows' says to put letters next to each other.
'right' becomes 'r' (common abbreviation).
'bearing' becomes 'e' (East, a compass bearing or direction).
'r'+'e'='re'
'ally' put after 're' is 'REALLY'.
(Other definitions for really that I've seen before include "Actually, in fact" , "are you sure?" , "Indeed" , "In truth - join forces again" , "Do you mean it?" .)