Unusual pattern covering southern part of church (8)
I believe the answer is:
transept
'part of church' is the definition.
('transept' is part of 'church')
'unusual pattern covering southern' is the wordplay.
'unusual' indicates anagramming the letters.
'covering' means one lot of letters goes inside another.
'southern' becomes 's' (abbreviation).
'pattern' with letters rearranged gives 'tranept'.
'tranept' placed around 's' is 'TRANSEPT'.
(Other definitions for transept that I've seen before include "Part of church intersecting nave" , "Patterns in part of church at right angle to nave" , "Arm of church body" , "Patterns found in parts of church" , "always running north to south" .)