Partly why bridge is crossed? (6)
I believe the answer is:
hybrid
'is crossed?' is the definition.
('hybrid' can be a synonym of 'crossed')
'partly why bridge' is the wordplay.
'partly' indicates the answer is hidden within the clue.
'HYBRID' is hidden in the letters of 'why bridge'.
(Other definitions for hybrid that I've seen before include "Cross of two different species or varieties" , "Plant that is the offspring of two different species" , "A composite of mixed origin" , "Offspring of mixed parents" , "Crossbreed or mixture" .)