Complex patterns in part of church (8)
I believe the answer is:
transept
'part of church' is the definition.
('transept' is part of 'church')
'complex patterns' is the wordplay.
'complex' indicates an anagram.
'patterns' is an anagram of 'TRANSEPT'.
'in' is the link.
(Other definitions for transept that I've seen before include "Part of church off aisle" , "Crossing part of a church" , "Side part of a church" , "Patterns in part of church at right angle to nave" , "Wing of cross-shaped church" .)