Wing of a church at right angles to the nave (8)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
transept
(Other definitions for transept that I've seen before include "Part of a church - patterns (anag)" , "Side part of a church" , "Wing of cross-shaped church" , "Section of church" , "Patterns found in parts of church" .)