Favour first of entrants in advance (6)
I believe the answer is:
before
'in advance' is the definition.
('before' can be a synonym of 'in advance')
'favour first of entrants' is the wordplay.
'favour' becomes 'befor' (to favour something is to be 'for' it).
'first of' indicates taking the first letters.
The first letter of 'entrants' is 'e'.
'befor'+'e'='BEFORE'
(Other definitions for before that I've seen before include "In advance" , "In front of (something)" , "Earlier than something" , "In front - earlier" , "On a previous occasion" .)