Archbishop one rarely missing party (6)
I believe the answer is:
anselm
'archbishop' is the definition.
(I've seen this before)
'one rarely missing party' is the wordplay.
'one' becomes 'an' (I've seen this in other clues**).
'rarely' becomes 'seldom' ('seldom' can be a synonym of 'rarely').
'missing' indicates named letters should be taken away.
'party' becomes 'do' (do is a kind of party).
'seldom' with 'do' taken out is 'selm'.
'an'+'selm'='ANSELM'
(Other definitions for anselm that I've seen before include "Sainted Archbishop of Canterbury" , "Canonised archbishop of Canterbury" , "Christian saint" , "Man" , "Italian monk, saint and Archbishop of Canterbury, 1093-1109" .)