Foremost of priests come to deliver a sermon (6)
I believe the answer is:
preach
'deliver a sermon' is the definition.
(I've seen this before)
'foremost of priests come' is the wordplay.
'foremost of' indicates taking the first letters.
'come' becomes 'reach' (reaching is a kind of coming).
The first letter of 'priests' is 'p'.
'p'+'reach'='PREACH'
'to' is the link.
(Other definitions for preach that I've seen before include "Speak from the pulpit" , "Publicly proclaim" , "Proclaim (a religious message)" , "Give moral or religious advice" , "Sermonise" .)