Part of church almost entirely unplanned (6)
I believe the answer is:
chance
'unplanned' is the definition.
('chance' can be similar in meaning to 'unplanned')
'part of church almost entirely' is the wordplay.
'part of church' becomes 'chancel' ('chancel' is part of 'church').
'almost entirely' means to remove the last letter.
'chancel' with its last letter taken away is 'CHANCE'.
(Other definitions for chance that I've seen before include "Coincidence" , "Serendipity" , "Opportunity; random effect" , "Risk the fortune" , "Possibility of something happening" .)