Archbishop is formal at start of evensong (7)
I believe the answer is:
primate
'archbishop' is the definition.
(thesaurus)
'formal at start of evensong' is the wordplay.
'formal' becomes 'prim' (I've seen this in other clues).
'start of' says to take the initial letters.
The first letter of 'evensong' is 'e'.
'prim'+'at'+'e'='PRIMATE'
'is' acts as a link.
(Other definitions for primate that I've seen before include "Mammal such as human" , "senior cleric" , "Highest order of mammal" , "A permit for a cardinal" , "C of E archbishop" .)