Part of a church at right angles to the nave (8)
I believe the answer is:
transept
'part of a church at right angles to the nave' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for transept that I've seen before include "Crossing part of a church" , "Patterns in part of church at right angle to nave" , "Part of a church at right angles to the nave" , "Cross-aisle in church" , "Part of church intersecting nave" .)