Boring race rehearsal (3,3)
I believe the answer is:
dry run
'rehearsal' is the definition.
('dry run' can be a synonym of 'rehearsal')
'boring race' is the wordplay.
'boring' becomes 'dry'.
'race' becomes 'run' (run is a kind of race**).
'dry'+'run'='DRY RUN'
(Other definitions for dry run that I've seen before include "Thirsty sort of rehearsal?" , "Dummy trial on thirsty trot?" , "A dummy test - sounds thirsty" , "Practice session, rehearsal" , "Kind of rehearsal like thirsty trot" .)