Join priest cutting piece of cake? (6)
I believe the answer is:
splice
'join' is the definition.
(splicing is a kind of joining)
'priest cutting piece of cake?' is the wordplay.
'priest' becomes 'p' (ecclesiastical abbreviation).
'cutting' indicates putting letters inside (some letters cut their way into a word).
'piece of cake?' becomes 'slice' (I've seen this in another clue).
'p' inserted inside 'slice' is 'SPLICE'.
(Other definitions for splice that I've seen before include "Knit" , "Join (rope or wedding couple)" , "Unite (strands of rope)" , "Weave (strands of rope) together" , "Join end to end" .)