Friends with everyone -- that is what's central to Christmas (6)
I believe the answer is:
allies
'friends' is the definition.
('ally' can be a synonym of 'friend')
'everyone that is what's central to christmas' is the wordplay.
'everyone' becomes 'all' (I've seen this before).
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'what's central' indicates the central letters.
'to christmas' becomes 'present' (I've seen this before).
The central letter of 'present' is 's'.
'all'+'ie'+'s'='ALLIES'
'with' acts as a link.
(Other definitions for allies that I've seen before include "Formally cooperating countries" , "Countries working together" , "They support and encourage each other" , "They help each other, confederates" , "They join and fight together" .)